Only iTunes 10.5 beta 2 utilizes the native UI framework on Lion. Curiously, iTunes 10.3 is a carbon application that violates Apple's own developer user interface guidelines--it uses a custom window rendering scheme that only makes it look kind of like a native cocoa app when running on OS 10.6. But iTunes 10.5 is an all cocoa rewrite; hence, it is completely 64bit and also uses the native Lion UI.framework.