So for those hoping to be able to jailbreak the device and install various widgets, this may not be possible.
Interestingly, it seems that the Apple TV is in fact running iOS; although this wasn't mentioned at all in the keynote.
According to Daring Fireball's John Gruber, "The device is running iOS, though, trust me. So its possible that some sort of SDK is coming in the future. But Im not going to hold my breath. Judging from the demo today, I think the fact that its running iOS under the hood is just an implementation detail."