The iPod blossomed for two reasons: content portability and piracy/theft. Unlike the iPod, Apple TV will require payment to view content on a big screen. Apple can't include DVD ripping tools in its official software so you're going to be limited to viewing content purchased from the iTunes store. This has the potential to make Apple TV be exponentially less successful than iPod.
The other problem is that there isn't a compelling consumer problem the Apple TV solves - whereas the iPod is orders of magnitude better than a CD player. I think that there's a good chance that Apple TV will be a dud. It will be a decent product but not a great product like iPod.
Anti-piracy technology like Audible Magic (free NY Times sub. req'd) is a huge friend of cable because the current cable business model (for programmers) is very profitable. The reason consumers shifted to digital downloads in the music world is because they could get stuff for free. If they can't, and piracy is really stopped, cable will continue to thrive and the alternatives will not be very good.
A friend recently relayed this anecdote:
I was talking to my mother this afternoon and she told me that my family just upgraded to all of Comcast's HD offerings. I then mentioned that a lot of "futurists" (aka blowhards who make up facts) think TV will all be the on the Web. Her response, "Why would anyone watch TV on the Web when there are all of these new wonderful high quality channels on Comcast?"
*A note on the headline: Led Zeppelin's original name was going to be "The New Yardbirds," but when The Who's Keith Moon told Jimmy Page that "it would probably go over like a lead zeppelin," they changed the name to spite him. The letter "a" was dropped so that it wouldn't be pronounced "leed." (Rolling Stone)