Index: wp-includes/js/thickbox/thickbox.css
===================================================================
--- wp-includes/js/thickbox/thickbox.css	(revision 17904)
+++ wp-includes/js/thickbox/thickbox.css	(working copy)
@@ -49,9 +49,8 @@
 	background: #ffffff;
 	z-index: 102;
 	color:#000000;
-	display:none;
 	text-align:left;
-	top:50%;
+	top:-9999px;
 	left:50%;
 	border: 1px solid #555;
 	-moz-box-shadow: rgba(0,0,0,1) 0 4px 30px;
Index: wp-includes/js/thickbox/thickbox.js
===================================================================
--- wp-includes/js/thickbox/thickbox.js	(revision 17904)
+++ wp-includes/js/thickbox/thickbox.js	(working copy)
@@ -268,8 +268,13 @@
 
 //helper functions below
 function tb_showIframe(){
+	offset_top = jQuery("#TB_window").offset().top;
+	if( offset_top < 0 ) {
+		adminbar_height = ( jQuery('body.admin-bar').length ) ? 28 : 0;
+		offset_top = 20 + adminbar_height + 'px';
+	}
 	jQuery("#TB_load").remove();
-	jQuery("#TB_window").css({display:"block"});
+	jQuery("#TB_window").css({display:"block", top: offset_top});
 }
 
 function tb_remove() {
Index: wp-admin/js/theme-preview.dev.js
===================================================================
--- wp-admin/js/theme-preview.dev.js	(revision 17904)
+++ wp-admin/js/theme-preview.dev.js	(working copy)
@@ -10,9 +10,10 @@
 
 		if ( tbWindow.size() ) {
 			tbWindow.width(w).height(h);
+			offset_top = tbWindow.offset().top;
 			$('#TB_iframeContent').width(w).height(h - 27);
 			tbWindow.css({'margin-left': '-' + parseInt((w / 2),10) + 'px'});
-			if ( typeof document.body.style.maxWidth != 'undefined' )
+			if ( typeof document.body.style.maxWidth != 'undefined' && offset_top > 0 )
 				tbWindow.css({'top':'30px','margin-top':'0'});
 		}
 	};
Index: wp-admin/js/media-upload.dev.js
===================================================================
--- wp-admin/js/media-upload.dev.js	(revision 17904)
+++ wp-admin/js/media-upload.dev.js	(working copy)
@@ -40,9 +40,10 @@
 
 		if ( tbWindow.size() ) {
 			tbWindow.width( W - 50 ).height( H - 45 - adminbar_height );
+			offset_top = tbWindow.offset().top;
 			$('#TB_iframeContent').width( W - 50 ).height( H - 75 - adminbar_height );
 			tbWindow.css({'margin-left': '-' + parseInt((( W - 50 ) / 2),10) + 'px'});
-			if ( typeof document.body.style.maxWidth != 'undefined' )
+			if ( typeof document.body.style.maxWidth != 'undefined' && offset_top > 0 )
 				tbWindow.css({'top': 20 + adminbar_height + 'px','margin-top':'0'});
 		};
 
Index: wp-admin/js/plugin-install.dev.js
===================================================================
--- wp-admin/js/plugin-install.dev.js	(revision 17904)
+++ wp-admin/js/plugin-install.dev.js	(working copy)
@@ -9,9 +9,10 @@
 
 		if ( tbWindow.size() ) {
 			tbWindow.width( W - 50 ).height( H - 45 - adminbar_height );
+			offset_top = tbWindow.offset().top;
 			$('#TB_iframeContent').width( W - 50 ).height( H - 75 - adminbar_height );
 			tbWindow.css({'margin-left': '-' + parseInt((( W - 50 ) / 2),10) + 'px'});
-			if ( typeof document.body.style.maxWidth != 'undefined' )
+			if ( typeof document.body.style.maxWidth != 'undefined' && offset_top > 0 )
 				tbWindow.css({'top': 20 + adminbar_height + 'px','margin-top':'0'});
 		};