I think there seems to be a lot of confusion when the words "HTPC" and "HD" are mentioned.
Outputting to a HDTV is easy, recording HD is not. Right now HTPC's are limited to recording OTA (ATSC), not cable (QAM). There are a couple QAM PCI cards out there right now, but there is no official driver support for them in Windows. I believe the QAM cards come with software that directly accesses them so the QAM features only work in that software (which is rarely HTPC friendly). I believe there is some QAM support in Linux but you can only tune in unencrypted channels, ie usually the same ones that you can get OTA (CBS, ABC, NBC, FOX, etc).
CableCards will bring encrypted QAM to the PC here in the next few months, but the only Microsoft OS to support it will be Vista, will never work in XP. It also looks doubtful CableCards will work in any opensource version of Linux (DRM issues). I am not aware either way on its ability to run on OSX. Furthermore there is evidence CableCards won't be available to the public thru retail or OEM channels. Only computers "certified" by CableLabs will be allowed to have them (at least at first).
Once all of this comes to pass, you can still use a DVI (or HDMI) video card to play back on your TV but the card will (very) likely need to be HDCP compliant, along with the TV. There is still a lot of gaps in information regarding this. But when you consider you will likely need to buy a prebuilt HTPC anyway, it may be moot as the company making the computer would have that all taken care of.
But here's to hoping for HD in DIY HTPC's in the future, cause I sure would prefer it to be that way!
