Perhaps I'm late to the game here, but I can't recall this ever being discussed on any of the sites I follow. It seems that at some point in the last few months Comcast has added a lot of network and cable TV shows to its on-demand service. Since discovering this for myself I've been told by others that Comcast has been advertising, on TV, the availability of these shows for quite some time, but because I never watch commercials, I was unaware.
I have to say that the HD selection is a bit disappointing as compared to the breadth of available SD titles, but it's no more off-balance than the current HD/SD programming ratio; surely this disparity will continue to decay, and eventually flip-flop, as more and more people get HDTVs and demand HD content.
Comcast/the network is the DVR
What's funny about this whole thing is that it isn't the on-demand element that excites me, but rather the fact that I can now offload onto Comcast the storage that would otherwise be hogging up my DVR, and in the case of HD television, that can amount to a serious chunk of space. Moreover, in a good number of situations, this will solve the I-only-have-two-tuners problem (never mind the fact that I have such a problem to begin with :P).
Another plus is that this service minimizes the amount of time I need to persuade the DVR to do what I want — the Comcast HD DVR (by Motorola) is the single worst piece of electronic equipment I've ever owned — anything that can reduce the time I spend each day fixing its mistakes is a good thing.
The only downside to this system that I've been able to deduce is the fact that you can't fast-forward beyond the slowest fast-forward speed (the 30-second trick breaks too), but this annoyance is somewhat mitigated by the fact that these shows have fewer and shorter commercials.