You should read this thread:http://thegreenbutton.com/forums/thread/333042.aspx
There are a bunch of motherboards shipping pre-populated with the necessary BIOS support (most likely by accident). The most significant issue is getting a hold of an activation key but if you read through the thread you'll see the various methods folks have gone through to get past this. The other major issue is that there's always a risk MS will come up with some method to disable the non-offical OCUR boards but that seems like a long way away.
On another note, as a long time CableCard PC owner (actually several, in addition to building several for others), I'd seriously consider waiting until the Hauppauge HD-PVR/Windows 7 support is fleshed out and go that route instead. With the Hauppauge solution you eliminate the need for a special motherboard, eliminate all the DRM crap tied to cablecard recordings (right now ANYTHING you record via cablecard is encrypted and you can't do crap with it, like compy to another PC or sync to a portable media player for example; there's even a risk you can lose your recordings if you change too much hardware in your box), all the headaches dealing with the cable company still being retards when activating a cablecard, and you won't lose the ability to rent pay-per-view stuff as you'll have a set top box with the hauppauge solution (you can't do any on-demand stuff with Cablecard). Oh yeah, and don't forget that there's still no SDV solution yet for PC's. One down side to the HD-PVR though is a minor loss in visual quality as you are re-encoding the analog video signal but we are talking about a very very small loss in quality that can not even be seen by most. Well, and that you'll need to have one set top box for each HD-PVR so it's not as "elegant" of a solution. Oh yeah, the HD-PVR recordings are also much smaller than via cablecard as they use h.264 compression (cablecard recordings use MPEG-2).