The third generation Apple TV is the exact same size with the same exact ports as the second gen, but it's got some big upgrades under the hood. As usual, iFixit's given it the full teardown treatment.
The biggest difference is obviously the ability to play 1080p video, but other hardware deviations from last generation an extra antenna for better signal strength and distance and the thermal pad going missing. Check out the full teardown over at iFixit. [iFixit]