Here's a rendering of the marketplace, complete with Zune playback controls on the bottom. It seems like this is going to be the "iTunes" of the Zuniverse, managing downloads and playlists and uploading music into the Zune.
Personally we thought they would have incorporated it into Windows Media Player 11, but I guess they didn't want to lock out WMP for third-party stores like Napster or Yahoo! Also, check out the C-list bands on there (no offense to the bands). Back when I worked at a Music startup, we could only get C-list bands as well, and we took the "we're totally indie" angle, just like Microsoft is doing now.