Enhanced cTorrent, a command line torrent client, does work on the ATV2. I was able to install it using the NitoTV interface and searching for Torrent, but this command should also work:
Code:
apt-get install com.onaips.blogspot.ctorrent
userguide:
http://www.rahul.net/dholmes/ctorrent/userguide.html
You have to SSH the .torrent file onto the ATV2, but it does work.
While the ATV2 has limited internal space, I could see this being used for files one or a few at a time, then transferring them to a NAS or computer after you're done seeding. Using only the ATV2 for the actual torrenting means you're using a lot less power than keeping your computer on.
What would be great is if there was a way to execute the commands from within XBMC. Maybe even make some kind of ctorrent GUI as an XBMC add-on (but that would be farther down the road). I know you can use Launcher to execute shell scripts on a PC, but it doesn't seem to work on ATV2 (unless I'm just doing it wrong).
Oh, and this also works on the iPhone/iPad/iPod touch. If you install it through cydia you should be able to use the Safari download plugin to load torrents directly through the browser instead of having to SSH them in. There's even a GUI for it called Torrentula (doesn't have all the options the CLI does. Sometimes you have to manually select a tracker for a multi-tracker .torrent if the first tracker is dead, or nothing happens) Great for when you're in a pinch and what you need is only available as a torrent.