I just bought my first "iTunes Plus" tracks from iTunes. These are the new 256-bit, non-DRM tracks at the same price as regular, 128-bit DRM-protected tracks. (The specific songs were 'one of these days' and 'sky starts falling' by Doves.)
Anyway, when I did an import of my music library, Media Center 12.0.421 imported the tracks, but did so as if they were generic non-ID3 music files. You only see the filename in the track title field ("10 One of These Days"), and artist/album/genre are not filled out for either of these songs, even though all the other new iTunes files I imported at the same time imported normally.
Attempting to play these tracks results in silence and temporary unresponsiveness, while I can still play my DRMmed iTunes (new and old) just fine. The same tracks play fine inside iTunes on the same computer. I have iTunes 7.6.1.9 installed (which includes QuickTime 7.4.1).
I haven't seen anything about iTunes Plus in the release notes for MC12...