The Apple TV, like the upcoming iPhone, runs a stripped-down version of OS X. By adding a few files to the onboard hard drive, other services can be run. This technique should be generalizable to running other software not intended or supported by Apple. Which means that, if you’re willing to get your hands dirty, you can get a really nice little computer for $300. If you have a monitor/TV that uses HDMI, it seems like a really good idea.
Apple is generally pretty relaxed about people hacking its devices. For instance, they have made no efforts to stop people from running Linux on iPods. Of course, their software isn’t open source, but they do take a moderate stance that seems to be good PR, especially since it’s possible to hinder but not really to prevent this type of hacking.