Actually there *should* be both satellite and digital cable tuner cards coming out sometime before we all grow old and die. Microsoft and DirecTV have announced hardware. I believe that the digital cable tuners are waiting for the two-way cable card standard to be ratified. (Cable Card currently only allows one-way communication) All this to say that the DRM on these cards is said to be Draconian at the least. And they probably won't be released until after Vista is released.
Sounds like these will only be availible through your satalite provider. I can't see them making these open for everyone to start stealing DirectTV. I know they have cards that can receive free satalite transmitions.
Tuners that meet the requirements can use cable card. It's an open standard. The TiVo Series 3 PVR already meets that requirement and you need to install 2 cable cards in it, one for each tuner. The new M-Cablecards (MultiChannel/MultiStream) are not out yet. Once you have a cable card in your TV or PVR, you no longer need the set top box, unless of course you want to use the movie on demand features. As stated, cable card is currently only unidirectional.