Cable Labs is really putting the screws on OEM users, who like to home-brew their media center PC on the cheap. To view and record HDTV content on a new Vista MCE PC, the following is a list of required components (thanks Chris Lanier):
1) Vista Home Premium or Ultimate
2) HDCP Video Card
3) Special BIOS w/ OCUR bit set
4) OCUR (CableCARD Reader/Digital Cable Tuner)
5) OCUR Product Key (separate from Windows Product Key)
6) Sign MS agreement
7) Sign CableLabs agreement
8) Video drivers that support COPP, PVP-OPM, CGMS-A
9) Sound card to support exact requirements of CableLabs
Thought: The OCUR product key worries me a little, how much does one want to bet that the product key is pre-installed with a new system and not made available to the end consumer - preventing upgrade/re-pave of the system.
So.. essentially you're looking at a completely new box - no hope of just adding cable card support to your existing MCE hardware. At present, very few vendors are providing Cable Labs certified PCs, and I haven't seen much in the way of certified bare-bones systems either.
According to Engadget, Vidabox are coming out with an all sing and dancing cable-card, Blue Ray, HD DVD unit in March, but it at $4500 it's not cheap:
http://www.engadget.com/2007/01/23/vidabox-rolls-cablecard-into-blu-ray-and-hd-dvd-all-in-one-media/
Conclusion: it is still early days. Vista isn't yet mainstream, and many vendors have yet to come out of the "Cable Labs" woodwork. I expect to see a change throughout this year as devices come onto the market that support cable card HDTV.