Broadcast TV may be in the process of being replaced by satellite and cable, but the program/advertiser link is as strong as ever (and more pervasive, as evidenced by the annoying ads that are now projected along the bottom of the screen
during programs). It's just going through a restructuring to deal with new elements like Tivo.
DVDs, broadband streaming, etc, are using the same basic selling methods, attaching ads to something you download, stream or buy, and in many cases making sure the content can't be viewed unless you sit through their ads. So the method is firmly entrenched, and jumping mediums as fast as a mosquito full of Barry Bonds' blood.
Jon, you're right: I'd expect e-book ads to contribute to a lower cost of e-book, the same way it did for commercial VHS movies, for example. If not, and riot ensues, I'll bring the Molotov cocktails.