Added new function-testing code that is completely cross-browser compatible.

src/jquery/jquery.js

@@ -1222,8 +1222,11 @@ jQuery.extend({
 			$ = jQuery._$;
 	},
 
+	// This may seem like some crazy code, but trust me when I say that this
+	// is the only cross-browser way to do this. --John
 	isFunction: function( fn ) {
-		return fn && typeof fn == "function";
+		return !!fn && typeof fn != "string" &&
+			typeof fn[0] == "undefined" && /function/i.test( fn + "" );
 	},
 
 	/**