At this point, it's impossible to say for sure what Apple will do with the ATV next. Some, myself included, think there is the possibility of seeing an App store for the existing ATV at some point in the near future (allowing access to additional content and games). If that is the case, I would imagine that we'll see the ATV joining the yearly cycle that all the other iOS devices seem to be on. In which case, well see a new ATV in September.
Other point out that it took years to see an ATV2, and that is a completely different box (from the hardware to the software a complete overhaul). If this proves to be the norm for the Apple TV, we won't be seeing one for quite some time.
I think Apple has a more specific vision for the ATV than it did before and that vision will benefit from yearly updates.