Anybody else using the Comcast app? I use it about as much as Netflix. It doesn't have quite the same library as the On-Demand system... there are some movies and shows that are only available on the iPad or only On-Demand on the cable box. However, I have found that the quality is nice... the interface is a little nicer than Netflix in some ways... worse in others. It will ONLY work on wifi.
There is one dangerous thing: there are lots of soft-core adult shows available (HBO, Cinemax, etc). Beware of the sticky iPad!
The Comcast app will also change channels on any of your cable boxes and will schedule DVR recordings, although you cannot watch what is on your DVR from the iPad. Starting an On-Demand show from the iPad (watching it on TV) requires you to press the enter/select button on the Comcast remote.
One thing I have noticed, however, is that the lower channel numbers are sometimes wrong on my iPad app. I think the app has me as being on the wrong Comcast network. I live right at the border between two different services, and it goes by zip code. I'm going to talk to Comcast and see if I can get that straightened out.