Index: core.js
===================================================================
--- core.js	(revision 5554)
+++ core.js	(working copy)
@@ -349,7 +349,9 @@
 	},
 
 	is: function( selector ) {
-		return !!selector && jQuery.multiFilter( selector, this ).length > 0;
+		return typeof selector == 'string' ? 
+			jQuery.multiFilter( selector, this ).length > 0 :
+			jQuery.inArray( selector, this ) != -1;
 	},
 
 	hasClass: function( selector ) {