Apple: 1080p Apple TV uses a Single-Core A5 SoCby Anand Lal Shimpi on March 7, 2012 2:51 PM EST
Details are scarce, but the new 1080p Apple TV does use a single-core Apple A5 SoC. I'm assuming the video decode engine is identical to what we have in the iPhone 4S today. The chassis design and remote remain identical to the previous 720p model.
Update: Brian beat me to it here with a much more detailed post on the single-core A5 and some discussion about the new iPad.