This Ceton Multi-Channel CableCARD is very interesting, both for its ability to decode six cable streams at once to record six shows at once on your Windows Media Center, and for the fact that it's not all that expensive.
Engadget got them to say that, depending on your order, the four-tuner version of the card will retail somewhere between $300 and $600. In comparison, an ATI Digital TV Tuner is $230, so four of those would bring you up to $920. And, you would need four separate CableCARDs from your cable service provider. which at $5 each, runs you an extra $20 on your bill. And the savings get better when you go to the 6 card version.
The official launch is 2010, so we won't be seeing these cards for a little while yet, so there's time for the networks to collaborate to make and air six shows that are worth recording in the same time slot. [Ceton via Engadget]