Index: src/core.js
===================================================================
--- src/core.js	(revision 5772)
+++ src/core.js	(working copy)
@@ -614,8 +614,28 @@
 
 	// See test/unit/core.js for details concerning this function.
 	isFunction: function( fn ) {
-		return !!fn && typeof fn != "string" && !fn.nodeName &&
-			fn.constructor != Array && /^[\s[]?function/.test( fn + "" );
+		return !!fn && ( !jQuery.browser.msie ? fn instanceof Function : 
+			(function(fn){
+				// IE specific test
+				if ( fn instanceof Function ) 
+					return true;
+				if ( typeof fn === "string" || fn instanceof String
+				 	|| fn.constructor === Array )
+					return false;
+				// Remove toString() and valueOf() overriden methods
+				// and dump the function as string
+				var s = "toString",
+					v = "valueOf",
+					t = typeof fn[s] === "function" && fn[s],
+					o = typeof fn[v] === "function" && fn[v],
+					r;
+				if ( t ) delete fn[s];
+				if ( o ) delete fn[v];
+				r = /^[\s[]?function/.test(fn + "");
+				if ( t ) fn[s] = t;
+				if ( o ) fn[v] = o;
+				return r;
+			})(fn) );
 	},
 
 	// check if an element is in a (or is an) XML document