The Apple TV jailbreak community remains in a holding pattern. As yet, the greenp0ison and limera1n jailbreak tools do not install command-line/full file access to the box, a precondition for most kinds of application development. That doesn't mean there aren't interesting events going on, however.
If you're a hardware hacker, iFixit has a great DIY project for you. Step 8 of their teardown suggests that it shouldn't be hard to solder on a standard iPod docking connector. Over at the #awkwardtv channel of irc.moofspeak.net, hobbyist davilla (of atv-bootloader/atv-creator fame) is attempting to make that hack a reality.
We'll report back on its success, should he succeed. The default Apple TV hardware does not allow you to use the
