Well we know it will enable AirPlay mirroring from the iPad 2, there have been hints of 1080p output as well. I wouldn't be surprised if there were some other new features as well. I was really hoping an AppleTV dev program was going to be announced at wwdc this year but no dice... Maybe next year?
Its probably worth noting that the current "iOS 5 Beta" for Apple TV has a build number of 4.4, suggesting we haven't actually seen iOS 5 for Apple TV. I'd expect new features around the autumn iPod event.