I just bought and installed the Flash Builder 4 after having the Beta for a while. The same code is giving me compile-time errors and suggests using mx1 instead of mx! If I make it mx1 the compile error goes away. Here is the top of my component.
<s:SkinnableContainer xmlns:fx="http://ns.adobe.com/mxml/2009"
xmlns:s="library://ns.adobe.com/flex/spark"
xmlns:mx="library://ns.adobe.com/flex/halo"
I mentioned nothing about mx1.