When = Soon, but difficult to say exactly when as each OEM works on finalizing their offerings. The Windows Media Center team is working with each to make sure these make it to market as soon as possible. (Yeah, I know -- this sounds like a total non-answer -- it's really the best information I have at the moment.)
Where = Dell, Sony, Velocity Media, Toshiba and Niveus Media have announced they will be selling CableCard equipped Windows Media Center systems. Take your pick and start monitoring those OEM websites for ordering details.