Index: includes/ajax.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/ajax.inc,v
retrieving revision 1.40
diff -u -p -r1.40 ajax.inc
--- includes/ajax.inc	17 Dec 2010 01:03:58 -0000	1.40
+++ includes/ajax.inc	21 Dec 2010 02:15:19 -0000
@@ -567,7 +567,7 @@ function ajax_pre_render_element($elemen
   }
 
   // Add a reasonable default event handler if none was specified.
-  if (isset($element['#ajax']) && !isset($element['#ajax']['event'])) {
+  if (!isset($element['#ajax']['event'])) {
     switch ($element['#type']) {
       case 'submit':
       case 'button':
@@ -602,6 +602,25 @@ function ajax_pre_render_element($elemen
     }
   }
 
+  // By default, prevent the browser from performing the default action of the
+  // event to which AJAX is binding. However, in Internet Explorer, preventing
+  // the default action of a radio or checkbox 'change' event prevents the
+  // element from being checked or unchecked.
+  // @todo For Drupal 8, revisit this logic. Perhaps not just radio and checkbox
+  //   elements, but everything other than buttons and links should default to
+  //   TRUE.
+  if (!isset($element['#ajax']['allow_default_action'])) {
+    switch ($element['#type']) {
+      case 'radio':
+      case 'checkbox':
+        $element['#ajax']['allow_default_action'] = TRUE;
+        break;
+
+      default:
+        $element['#ajax']['allow_default_action'] = FALSE;
+    }
+  }
+
   // Attach JavaScript settings to the element.
   if (isset($element['#ajax']['event'])) {
     $element['#attached']['library'][] = array('system', 'jquery.form');
Index: misc/ajax.js
===================================================================
RCS file: /cvs/drupal/drupal/misc/ajax.js,v
retrieving revision 1.33
diff -u -p -r1.33 ajax.js
--- misc/ajax.js	17 Dec 2010 01:03:58 -0000	1.33
+++ misc/ajax.js	21 Dec 2010 02:15:21 -0000
@@ -111,7 +111,8 @@ Drupal.ajax = function (base, element, e
     },
     submit: {
       'js': true
-    }
+    },
+    allow_default_action: false
   };
 
   $.extend(this, defaults, element_settings);
@@ -221,7 +222,7 @@ Drupal.ajax.prototype.eventResponse = fu
 
   // Do not perform another ajax command if one is already in progress.
   if (ajax.ajaxing) {
-    return false;
+    return ajax.element_settings.allow_default_action;
   }
 
   try {
@@ -250,15 +251,7 @@ Drupal.ajax.prototype.eventResponse = fu
     alert("An error occurred while attempting to process " + ajax.options.url + ": " + e.message);
   }
 
-  // For radio/checkbox, allow the default event. On IE, this means letting
-  // it actually check the box.
-  if (typeof element.type != 'undefined' && (element.type == 'checkbox' || element.type == 'radio')) {
-    return true;
-  }
-  else {
-    return false;
-  }
-
+  return ajax.element_settings.allow_default_action;
 };
 
 /**