It was known that Jailbreak was possible (I remember someone report PwnageTools works on JB-ing ATV on pre-patched firmware)
Now limera1n is known by reports from users that DOES JB the ATV.

Not so excited yet tho, since no OpenSSH nor APT works “yet”, there is not much room for hacking yet….
Since one cannot “install” nor “execute” any Apps on the ATV now.

P.S.:
A bit of corrections, yes there are way to “install” apps on ATV even now, but since there is no Springboard (Launcher Like) UI, and there is no SSH yet, there is no way of executing the application yet..
But since the hard part has been passed (JB), hopes are high and I think that should be just around the corner… (I want lighttpd+PHP+Perl+SQLite and AirVideo on ATV!!!)

Links:
– TUAW – limera1n jailbreak released, greenpois0n jailbreak delayed
– Geeky-gadgets – Limera1n Jailbreak Works On Apple TV