Here's what I found on the Microsoft website regarding Vista Media Center:
TV tuners and remotes
If you're a serious TV fan, you can use Windows Media Center with more than one TV tuner.1 Then you can watch one show while recording another or record two shows at the same time. You can even record two shows simultaneously while watching a third previously recorded show! An optional remote also adds intuitive, streamlined navigation and control to your TV viewing experience.
1 Windows Media Center supports as many as two NTSC Standard tuners and two ATSC HD TV tuners, for a maximum of four TV tuners in a Windows Media Center PC.
If VMC will support more tuners than that you'd think they'd list it on the official website. Even with four tuners it still falls short of my needs. This is exactly why Microsoft should stick with creating a basic OS (or at least try and create one that works the way it's supposed to without all the bugs) and leave the extra bells and whistles to the aftermarket companies that can get it right. I think I'll stick with BeyondTV for now. After all, if it ain't broke....