We've heard a little bit more about the upcoming Comcast video store hitting PCs and set-top-boxes before October '06.
On the PC side, the files are going to be encoded in mp4/m4p and Flash format, which means you can either stream your "paid for" movies directly from either a browser or a third party app, or download the mp4/m4p to watch on your computer, phone, or iPod. On the set-top-box side, the files are going to be encoded in some sort of MPEG (2 or 4, we're not sure), and downloaded onto your box to watch.
The interesting news is that when a download is started, because you're on Comcast's ISP downloading from Comcast's own media servers, they'll bump up your download cap to whatever they want, guaranteeing you get your data on time and that there are no stutters in the video. This may go into the grey area of Net Neutrality, so we'll watch and see where this plan is going to go.