You should be waiting for pwnagetool which is due this Sunday, fingers crossed. This will let you cook a firmware to restore to the Apple TV which includes utilities necessary to gain access to it.
Probably wont be much use until an interface for downloading Apps is available. At which point you'll most probably have to cook another firmware and restore again.
If you can find your way around a command line there may be some things from the iPhone/iPad world which will "just work" on the Apple TV. But they're not going to be terribly exciting.
A command-line version of mplayer, for example, may successfully output to the TV right away (if there is one!)... but only time will tell.
