Interesting tidbit - 1080p rendering (via TV out) might have been added in iOS 5, and only for new hardware, but iOS 4.0 actually enabled playback of 1080p video files (albeit scaled to screen resolution) even on the iPhone 3GS - I know, because I've tried it...(the iPhone 3G on the other hand was completely incapable of playing 1080p, so it's 3GS and later hardware)

iTunes wouldn't let you sync a 1080p clip onto an iPhone at the time, but if you used any of those Wifi file sharing apps to get the file across that also had media playback capability, the 3GS would happily decode and play a 10 Mbit/sec 1080p 6 audio channel H.264 file - scaling it to SD and stereo of course, but impressive nevertheless. From memory Apple disabled this functionality around iOS 4.1 since it was undocumented, and not entirely stable. (Attempting to play certain video files could lock up the sound system on the phone requiring a reboot to get sound back!)

From that I'm sure the more powerful current ATV2 must have the horsepower to play back 1080p files, albeit probably scaling the actual output to the TV down to 720p. The rumored ATV3 with an A5 would no doubt allow for actual 1080p output - considering that the iPad 2 already has this ability with some games like Real Racing 2 HD already supporting true 1920x1080 output in second screen mode. (As opposed to just scaling up 1024x768 in mirroring mode)