@scubadave - If you are already have Netflix, you won't need to rent so many movies & TV shows from iTunes.
Also, I fully expect Hulu+ to be on it within 6 months, along with an explosion of apps. (WWDC 2011 at the latest!) Like the original iPhone, it took some time before the dev tools were made available for 3rd party apps. I think the primary reasons for this was to mature the tools and documentation, but most importantly for developers to experience the Cocoa Touch UI approach and change their thinking prior to them porting desktop designs over. (Windows Mobile anyone?)
The AppleTV is iOS based, but the UI layer obviously isn't multitouch, and there need to be different interaction paradigms. Apple would want to get people used to their "Cocoa TV"-based apps, so the apps get a consistent quality, look and feel. Most of iOS is mature, but Apple will probably need to spend time cleaning up the new parts before opening them up.
Now, I suspect that jailbreakers get apps within a month, perhaps XBMC or Boxee will make an appearance! (Drool from a guy is has no interest in jailbreaking his iPhone here)
I'm kind of split between the AppleTV and Roku. As of today, I think the Roku has better value, but the AppleTV has better potential value. I could buy both for less than a Boxee Box, or three months of cable. Almost disposable!
The HTPC route is interesting, but I don't think will be a good fit, and a bit more expensive.