Can you set it up to automatically start with XBMC? How hard/easy is it to upgrade XBMC as new versions come out? Does the AppleTV support a keyboard or some other device (iPhone?) to control ATV?
2. No idea. If I had to guess it wouldn't be harder than the original jailbreak, and potentially much easier.
3. You can use your iPhone/iPod touch's Remote app to control the ATV (the remote it comes with is a pain for text input) [No jailbreak required]