diff --git a/user/src/com/google/gwt/jsonp/client/JsonpRequest.java b/user/src/com/google/gwt/jsonp/client/JsonpRequest.java
index ad8d5d4..c199339 100644
--- a/user/src/com/google/gwt/jsonp/client/JsonpRequest.java
+++ b/user/src/com/google/gwt/jsonp/client/JsonpRequest.java
@@ -32,32 +32,47 @@
 public class JsonpRequest<T> {
 
   /**
-   * Each request will be assigned a new id.
+   * A global JS variable that holds the next index to use.
    */
-  private static int callbackCounter = 0;
+  @SuppressWarnings("unused") // accessed from JSNI
+  private static final String CALLBACKS_COUNTER_NAME = "__gwt_jsonp_counter__";
 
   /**
-   * __gwt_jsonp__ is a global object that contains callbacks of pending requests.
+   * A global JS object that contains callbacks of pending requests.
    */
   private static final String CALLBACKS_NAME = "__gwt_jsonp__";
-  private static final JavaScriptObject CALLBACKS = createCallbacksObject(CALLBACKS_NAME);
+  private static final JavaScriptObject CALLBACKS = getOrCreateCallbacksObject();
   
   /**
-   * Creates a global object to store callbacks of pending requests.
-   *
-   * @param name The name of the global object.
-   * @return The created object.
+   * @return the next ID to use, incrementing the global counter.
    */
-  private static native JavaScriptObject createCallbacksObject(String name) /*-{
-    return $wnd[name] = new Object();
+  private static native int getAndIncrementCallbackCounter() /*-{
+    var name = @com.google.gwt.jsonp.client.JsonpRequest::CALLBACKS_NAME;
+    var ctr = @com.google.gwt.jsonp.client.JsonpRequest::CALLBACKS_COUNTER_NAME;
+    return $wnd[name][ctr]++;
   }-*/;
   
   private static Node getHeadElement() {
     return Document.get().getElementsByTagName("head").getItem(0);
   }
   
+  /**
+   * @return a global object to store callbacks of pending requests, creating
+   * it if it doesn't exist.
+   */
+  private static native JavaScriptObject getOrCreateCallbacksObject() /*-{
+    var name = @com.google.gwt.jsonp.client.JsonpRequest::CALLBACKS_NAME;
+    if (!$wnd[name]) {
+      $wnd[name] = new Object();
+      $wnd[name]
+          [@com.google.gwt.jsonp.client.JsonpRequest::CALLBACKS_COUNTER_NAME]
+          = 0;
+    }
+    return $wnd[name];
+  }-*/;
+
   private static String nextCallbackId() {
-    return "I" + (callbackCounter++);
+    return "I" + getAndIncrementCallbackCounter();
   }
 
   private final String callbackId;
@@ -120,6 +135,16 @@
     return timeout;
   }
 
+  @Override
+  public String toString() {
+    return "JsonpRequest(id=" + callbackId + ")";
+  }
+
+  // @VisibleForTesting
+  String getCallbackId() {
+    return callbackId;
+  }
+
   /**
    * Sends a request using the JSONP mechanism.
    * 
diff --git a/user/test/com/google/gwt/jsonp/client/JsonpRequestTest.java b/user/test/com/google/gwt/jsonp/client/JsonpRequestTest.java
index 8022eb7..0d0cc69 100644
--- a/user/test/com/google/gwt/jsonp/client/JsonpRequestTest.java
+++ b/user/test/com/google/gwt/jsonp/client/JsonpRequestTest.java
@@ -16,7 +16,10 @@
 package com.google.gwt.jsonp.client;
 
 import com.google.gwt.core.client.GWT;
+import com.google.gwt.junit.DoNotRunWith;
+import com.google.gwt.junit.Platform;
 import com.google.gwt.junit.client.GWTTestCase;
+import com.google.gwt.user.client.Timer;
 import com.google.gwt.user.client.rpc.AsyncCallback;
 
 /**
@@ -25,16 +28,6 @@
 public class JsonpRequestTest extends GWTTestCase {
 
   /**
-   * The maximum amount of time to wait for a response in milliseconds.
-   */
-  private static final int RESPONSE_DELAY = 2500;
-
-  /**
-   * The current Id of the async callback.
-   */
-  protected static int currentId;
-
-  /**
    * Checks that an error is received.
    */
   private class AssertFailureCallback<T> implements AsyncCallback<T> {
@@ -42,19 +35,19 @@
     private int id;
 
     public AssertFailureCallback(String expectedMessage) {
-      id = ++currentId;
+      id = currentTestId;
       this.expectedMessage = expectedMessage;
     }
 
     public void onFailure(Throwable throwable) {
-      if (id == currentId) {
+      if (id == currentTestId) {
         assertEquals(expectedMessage, throwable.getMessage());
         finishTest();
       }
     }
 
     public void onSuccess(T value) {
-      if (id == currentId) {
+      if (id == currentTestId) {
         fail();
       }
     }
@@ -64,24 +57,34 @@
    * Checks that the received value is as expected.
    */
   private class AssertSuccessCallback<T> implements AsyncCallback<T> {
-    private T expectedValue;
-    private int id;
+    private final T expectedValue;
+    private final int id;
+    private final Counter counter;
 
     private AssertSuccessCallback(T expectedValue) {
-      this.id = ++currentId;
+      this(expectedValue, null);
+    }
+
+    private AssertSuccessCallback(T expectedValue, Counter counter) {
+      this.id = currentTestId;
+      this.counter = counter;
       this.expectedValue = expectedValue;
     }
 
     public void onFailure(Throwable throwable) {
-      if (id == currentId) {
+      if (id == currentTestId) {
         fail();
       }
     }
 
     public void onSuccess(T value) {
-      if (id == currentId) {
+      if (id == currentTestId) {
         assertEquals(expectedValue, value);
-        finishTest();
+        if (counter != null) {
+          counter.count();
+        } else {
+          finishTest();
+        }
       }
     }
   }
@@ -100,10 +103,51 @@
     }
   }
 
+  /**
+   * A class that counts results and calls finishTest when the right number
+   * have been received.
+   */
+  private class Counter {
+  
+    private int count;
+
+    public Counter(int count) {
+      this.count = count;
+    }
+  
+    public void count() {
+      if (--count < 0) {
+        fail("Too many results");
+      }
+      if (count == 0) {
+        finishTest();
+      }
+    }
+  }
+
+  /**
+   * The maximum amount of time to wait for a response in milliseconds.
+   */
+  private static final int RESPONSE_DELAY = 2500;
+
+  /**
+   * The ID of the current test.
+   */
+  protected static int currentTestId;
+
   private static String echo(String value) {
     return GWT.getModuleBaseURL() + "echo?action=SUCCESS&value=" + value;
   }
 
+  private static String echoDelayed(String value) {
+    return echoDelayed(value, 500);
+  }
+
+  private static String echoDelayed(String value, long delayMs) {
+    return GWT.getModuleBaseURL() + "echo?action=SUCCESS&delay=" + delayMs
+       + "&value=" + value;
+  }
+
   private static String echoFailure(String error) {
     return GWT.getModuleBaseURL() + "echo?action=FAILURE&error=" + error;
   }
@@ -131,6 +175,31 @@
         Boolean.TRUE));
   }
 
+
+  @DoNotRunWith(Platform.HtmlUnit)
+  // Fails in devmode with HtmlUnit, JS "null" exception
+  public void testCancel() {
+    delayTestFinish(2000);
+    // setup a server request that will delay for 500ms
+    JsonpRequest<String> req = jsonp.requestString(echoDelayed("A", 500),
+        new AssertFailureCallback<String>("A"));
+    // cancel it before it comes back
+    req.cancel();
+    // wait 1s to make sure we don't get a callback
+    new Timer() {
+      @Override
+      public void run() {
+        finishTest();
+      }      
+    }.schedule(1000);
+  }
+
+  public void testDelay() {
+    delayTestFinish(RESPONSE_DELAY);
+    JsonpRequest<String> req = jsonp.requestString(echoDelayed("'A'"),
+        new AssertSuccessCallback<String>("A"));
+  }
+
   public void testDouble() {
     delayTestFinish(RESPONSE_DELAY);
     jsonp.requestDouble(echo("123.456"), new AssertSuccessCallback<Double>(
@@ -144,6 +213,20 @@
         new AssertFailureCallback<String>("ERROR"));
   }
 
+  @DoNotRunWith(Platform.HtmlUnit)
+  // Hangs indefinitely in devmode with HtmlUnit
+  public void testIds() {
+    delayTestFinish(RESPONSE_DELAY);
+    JsonpRequest<String> reqA = jsonp.requestString(echo("'A'"),
+        new AssertSuccessCallback<String>("A"));
+    JsonpRequest<String> reqB = jsonp.requestString(echo("'B'"),
+        new AssertSuccessCallback<String>("B"));
+    // WARNING: knows the current structure of IDs
+    int idA = Integer.parseInt(reqA.getCallbackId().substring(1));
+    int idB = Integer.parseInt(reqB.getCallbackId().substring(1));
+    assertEquals("Unexpected ID sequence", idA + 1, idB);
+  }
+
   public void testInteger() {
     delayTestFinish(RESPONSE_DELAY);
     jsonp.requestInteger(echo("123"), new AssertSuccessCallback<Integer>(123));
@@ -180,6 +263,19 @@
     jsonp.requestString(echo("null"), new AssertSuccessCallback<String>(null));
   }
 
+  @DoNotRunWith(Platform.HtmlUnit)
+  // Hangs indefinitely in devmode with HtmlUnit
+  public void testOverlapped() {
+    delayTestFinish(RESPONSE_DELAY);
+    Counter counter = new Counter(3);
+    JsonpRequest<String> reqA = jsonp.requestString(echoDelayed("'A'", 750),
+        new AssertSuccessCallback<String>("A", counter));
+    JsonpRequest<String> reqB = jsonp.requestString(echoDelayed("'B'", 500),
+        new AssertSuccessCallback<String>("B", counter));
+    JsonpRequest<String> reqC = jsonp.requestString(echo("'C'"),
+        new AssertSuccessCallback<String>("C", counter));
+  }
+
   public void testString() {
     delayTestFinish(RESPONSE_DELAY);
     jsonp.requestString(echo("'Hello'"), new AssertSuccessCallback<String>(
@@ -202,4 +298,9 @@
     jsonp = new JsonpRequestBuilder();
     jsonp.setTimeout(RESPONSE_DELAY + 500);
   }
+  
+  @Override
+  protected void gwtTearDown() throws Exception {
+    ++currentTestId;
+  }
 }
diff --git a/user/test/com/google/gwt/jsonp/server/EchoServlet.java b/user/test/com/google/gwt/jsonp/server/EchoServlet.java
index 18461dc..7e8cef2 100644
--- a/user/test/com/google/gwt/jsonp/server/EchoServlet.java
+++ b/user/test/com/google/gwt/jsonp/server/EchoServlet.java
@@ -17,7 +17,6 @@
 
 import java.io.IOException;
 
-import javax.servlet.ServletException;
 import javax.servlet.http.HttpServlet;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
@@ -47,9 +46,11 @@
 
   @Override
   protected void service(HttpServletRequest req, HttpServletResponse res)
-      throws ServletException, IOException {
+      throws IOException {
+    res.setContentType("application/javascript");
     switch (Action.valueOf(req.getParameter("action"))) {
       case SUCCESS: {
+        handleDelay(req);
         String callback = req.getParameter("callback");
         String value = req.getParameter("value");
         if (value == null) {
@@ -61,6 +62,7 @@
       }
 
       case FAILURE: {
+        handleDelay(req);
         String failureCallback = req.getParameter("failureCallback");
         String error = req.getParameter("error");
         if (failureCallback != null) {
@@ -80,4 +82,21 @@
         // Don't respond anything so that a timeout happens.
     }
   }
+
+  /**
+   * Handle a delay query parameter if present.
+   * 
+   * @param req
+   */
+  private void handleDelay(HttpServletRequest req) {
+    String delayVal = req.getParameter("delay");
+    if (delayVal != null) {
+      int delayMs = Integer.parseInt(delayVal);
+      try {
+        Thread.sleep(delayMs);
+      } catch (InterruptedException e) {
+        log("Interrupted sleep, continuing");
+      }
+    }
+  }
 }