try using tricker opacity thingy

bootstrap-1.1.1.css

@@ -6,7 +6,7 @@
  * http://www.apache.org/licenses/LICENSE-2.0
  *
  * Designed and built with all the love in the world @twitter by @mdo and @fat.
- * Date: Sun Aug 28 16:34:52 PDT 2011
+ * Date: Sun Aug 28 17:40:04 PDT 2011
  */
 /* Reset.less
  * Props to Eric Meyer (meyerweb.com) for his CSS reset file. We're using an adapted version here	that cuts out some of the reset HTML elements we will never need here (i.e., dfn, samp, etc).
@@ -1679,8 +1679,10 @@ button.btn::-moz-focus-inner, input[type=submit].btn::-moz-focus-inner {
   box-shadow: inset 0 1px 1px rgba(0, 0, 0, 0.05);
 }
 .modal-backdrop {
-  background-color: rgba(0, 0, 0, 0.5);
+  /*  background-color: rgba(0,0,0,.5);
-  filter: progid:DXImageTransform.Microsoft.gradient(GradientType=0,startColorstr='#aa000000',endColorstr='#aa000000');
+  filter: e(%("progid:DXImageTransform.Microsoft.gradient(GradientType=0,startColorstr=%d,endColorstr=%d)", '#aa000000', '#aa000000'));*/
+
+  background-color: #000;
   position: fixed;
   top: 0;
   left: 0;
@@ -1688,6 +1690,9 @@ button.btn::-moz-focus-inner, input[type=submit].btn::-moz-focus-inner {
   bottom: 0;
   z-index: 1000;
 }
+.modal-backdrop.fade.in {
+  opacity: .5;
+}
 .modal {
   position: fixed;
   top: 50%;

bootstrap-1.1.1.min.css

@@ -226,7 +226,7 @@ button.btn::-moz-focus-inner,input[type=submit].btn::-moz-focus-inner{padding:0;
 .pagination ul li.disabled a,.pagination ul li.disabled a:hover{background-color:transparent;color:#bfbfbf;}
 .pagination ul li.next a{border:0;}
 .well{background-color:#f5f5f5;margin-bottom:20px;padding:19px;min-height:20px;border:1px solid #eee;border:1px solid rgba(0, 0, 0, 0.05);-webkit-border-radius:4px;-moz-border-radius:4px;border-radius:4px;-webkit-box-shadow:inset 0 1px 1px rgba(0, 0, 0, 0.05);-moz-box-shadow:inset 0 1px 1px rgba(0, 0, 0, 0.05);box-shadow:inset 0 1px 1px rgba(0, 0, 0, 0.05);}
-.modal-backdrop{background-color:rgba(0, 0, 0, 0.5);filter:progid:DXImageTransform.Microsoft.gradient(GradientType=0,startColorstr='#aa000000',endColorstr='#aa000000');position:fixed;top:0;left:0;right:0;bottom:0;z-index:1000;}
+.modal-backdrop{background-color:#000;position:fixed;top:0;left:0;right:0;bottom:0;z-index:1000;}.modal-backdrop.fade.in{opacity:.5;}
 .modal{position:fixed;top:50%;left:50%;z-index:2000;width:560px;margin:-280px 0 0 -250px;background-color:#ffffff;border:1px solid #999;border:1px solid rgba(0, 0, 0, 0.3);*border:1px solid #999;-webkit-border-radius:6px;-moz-border-radius:6px;border-radius:6px;-webkit-box-shadow:0 3px 7px rgba(0, 0, 0, 0.3);-moz-box-shadow:0 3px 7px rgba(0, 0, 0, 0.3);box-shadow:0 3px 7px rgba(0, 0, 0, 0.3);-webkit-background-clip:padding-box;-moz-background-clip:padding-box;background-clip:padding-box;}.modal .modal-header{border-bottom:1px solid #eee;padding:5px 20px;}.modal .modal-header .close{position:absolute;right:10px;top:10px;color:#999;line-height:10px;font-size:18px;}
 .modal .modal-body{padding:20px;}
 .modal .modal-footer{background-color:#f5f5f5;padding:14px 20px 15px;border-top:1px solid #ddd;-webkit-border-radius:0 0 6px 6px;-moz-border-radius:0 0 6px 6px;border-radius:0 0 6px 6px;-webkit-box-shadow:inset 0 1px 0 #ffffff;-moz-box-shadow:inset 0 1px 0 #ffffff;box-shadow:inset 0 1px 0 #ffffff;zoom:1;margin-bottom:18px;margin-bottom:0;}.modal .modal-footer:before,.modal .modal-footer:after{display:table;content:"";}

examples/assets/js/bootstrap-modal.js

@@ -107,13 +107,10 @@
         , animate = this.$element.hasClass('fade') ? 'fade' : ''
       if ( this.isOpen && this.settings.backdrop ) {
         this.$backdrop = $('<div class="modal-backdrop ' + animate + '" />')
+          .click(function () { that.close() })
           .appendTo(document.body)
-        $('body').delegate('.modal-backdrop', 'click.modal.backdrop', function () {
-          that.close()
-        })
       } else if ( !this.isOpen && this.$backdrop ) {
         this.$backdrop.removeClass('in')
-        $('body').undelegate('click.modal.backdrop')
 
         function removeElement() {
           that.$backdrop.remove()

lib/patterns.less

@@ -578,8 +578,12 @@ input[type=submit].btn {
 // ------
 
 .modal-backdrop {
-  background-color: rgba(0,0,0,.5);
+/*  background-color: rgba(0,0,0,.5);
-  filter: e(%("progid:DXImageTransform.Microsoft.gradient(GradientType=0,startColorstr=%d,endColorstr=%d)", '#aa000000', '#aa000000'));
+  filter: e(%("progid:DXImageTransform.Microsoft.gradient(GradientType=0,startColorstr=%d,endColorstr=%d)", '#aa000000', '#aa000000'));*/
+  background-color:#000;
+  &.fade.in {
+    opacity: .5;
+  }
   position: fixed;
   top: 0;
   left: 0;