CEPro has an article announcing that Microsoft is adding support for four CableCards and also announcing partners for Media Center Extender.
For those of you unfamiliar with CableCard - it is a device that allows a home theater PC to record encrypted content from Cable Systems. It is extremely locked down and has lots of copy-protection to keep you from using the content as you might want to, but it does allow the time-shifting of those encrypted channels. Previously, the Vista Media Center could support no more than two CableCards.
As far as Media Center Extenders, this has been a sore spot for many as the only extender device you could use with your Vista Media Center was an Xbox360 - you know the noisy, prone-to-break game box that wasn't really made to be an extender. :This will be very good news for those with the Vista Media Center.
UPDATE: After reading Chris Lanier's Post about this, I realized I failed to point out that this four-Cablecard tuner system will only be available to custom installers. Bummer for everyone (except the installer)! That really takes away from this news for the average joe like myself.