the New Zune Windows client is not WPF based. what the hell were they thinking?
my only guess would be to make it easier to get the UI to MediaCenter and Xbox 360s ... but in that regard, MS should be pushing to get better WPF/Silverlight support into MediaCenter, Xbox 360, and MediaRoom. not pushing MCML to the desktop, outside of MCE. remember, you're not a real UI framework until Charles Petzold writes a book about you