Are there any apps on Cydia specific to AppleTV?
Perhaps pedantically, I'd like to point out that while you can play back 1080p video through an Apple TV 2, you will still get 720p output. From the XBMC FAQ:


  At present, though the ATV2 can decode
  1080p video (using Apple's private
  VideoToolBox API for
  hardware-accelerated decoding), it is
  limited to 720p output. The 1080p
  video is decoded, then down-scaled in
  hardware to 720p.


Slightly off-topic, but I find Plex to be a superior Apple TV client to XBMC. I prefer the Plex interface to XBMC. You will, however, need to set up a Plex server, as Plex for iOS is a client that needs to hook up to a server, which does media management and transcoding as needed.