I hear that people who jailbreak the Apple TV 2 and install Plex can play 1080p videos, but the Apple TV 2 will not play them reliably. They'll sometimes play smoothly and start having issues at other times. I hear some people can play most of their 1080p videos while others can hardly play any at all. The Apple TV hardware simply isn't quite powerful enough to decode 1080p video without some playback issues.
The ideal conditions would to have a gigabit wired connection playing h.264-encoded mp4 files (since Apple hardware and software seems to like that video format the best). You may be able to play 1080p videos under those conditions, but don't expect it to work reliably.
There will be a future Apple TV 2 (probably in 2012) that has the hardware to play 1080p videos, but you will have to wait patiently for that one.
The Roku XPS is about the same price as the Apple TV can play 1080p video, and I know there is a version of the Plex client for the Roku, so perhaps you might want to try that. You would still need a Plex server to do the streaming to the device, but that's the same deal with Plex for the Apple TV. I do believe, however, that Plex for the Roku is fairly recent and is in a beta stage, so do your research if you decide to go with that.