As you know, the 2nd gen iOS Apple TV has already been jailbroken. While tools have not yet been released, you can watch a video posted earlier today showing command line access to the unit.
I do not have jb access so have limited my explorations to the tools I have available. I used Kevin Ballard's superb Bonjour Browser to scan for Apple TV services. At this time, there appear to be two. First, there is the standard AirTunes "raop" service. RAOP stands for the remote audio output protocol, which has been used for some time with Airport audio streaming. There's a nice discussion of the protocol over at CocoaDev.