I gave this a try last night. Hit a couple of snags; I was already running a webserver on port 80, so had to kill that, then got an error about port 53. managed to munge my DNS, but restored a file back (thanks time machine!) and got it all working.
It's not too user friendly at the moment, having to edit a pref file and then run via terminal, but I'm sure an app wrapper will appear shortly to fix that.
When you think about what is being done the question of "why didn't anyone think of that sooner?" pops up.
I purchased an ATV 3rd Gen shortly after they came out, mainly with the idea of putting XBMC on it, as I had done with my ATV 1, so I can watch / listen to the existing media I have in the format they are already in, and not have to re-encode / re-rip so that I can then add it to iTunes (if only adding the reference to the Library and not the file itself).
As we know, that isn't currently possible, nor likely to be any time soon, so I started to use Plex with my 'smart' Blu-Ray player to get similar functionality.