The ATVs don't tend to see as frequent changes as Apple's other products. I doubt ATV will see any major updates until Apple decides to radically change things, like integrating the set-top box with a TV or adding some kind of gaming ability. The only real change between the ATV2 and 3 was the addition of 1080p capability. Other than that, the two are pretty much the same.
Most of the updates have been on the software side, which has been great for ATV2 and 3 owners. Personally, I'm hoping for some additional channels -- most notably Amazon Prime -- though I think HBO Go will probably be the next one to appear.