FireCore has posted a status update on the state of the Apple TV 3 jailbreak.
As you may recall we were very optimistic about the ATV3 when it originally launched back in March, and while this optimism remains unchanged it's clear things have not progressed as quickly as anyone would have liked. The biggest hurdle that has yet to be overcome is also the most essential - the jailbreak. While we tend to think of ourselves as pretty resourceful, the security related expertise that is required in order to release a jailbreak for iOS based devices like the ATV is just not in our wheelhouse, and we're simply left to encourage the capable few as best we can. No one is more anxious than us to get the ATV3 opened up to 3rd party apps, but due to the reasons mentioned above we're not able to provide any kind of release timeline right now.
FireCore notes that aTV Flash for the ATV1 and ATV2 is still available. Those devices can be found on Amazon but for quite a premium.