Continuing my quest for a decent media center approach, I noticed that Brad Wilson is using MCE Tunes to stream iTunes content using Windows Media Center. (Also looks like he built his media center for the same reason I'm building mine - as a movie jukebox.) Looks like MCE Tunes also works over media center extenders (Xbox 360), so that'd be a good solution to the music portion of the media center.
I'll have to add this to my list of things to check out as I continue my quest. Unfortunately it only really works in the XP edition of media center right now, so it'd mean I can't use Vista as my media center. Not a showstopper, but definitely something to consider.