Quiqueck, on the Plex forums, has devised a method to install the media center on Apple TV as a plugin. It’s not exactly as easy as installing “regular” plugins—you’ll have to be comfortable with the Terminal—but I trust you can handle it.
Of course, your Apple TV has to be jailbroken for this to work.
As Wired is careful to point out, Apple TV already does some of what Plex does, namely stream videos from Mac to TV. True, but does the Apple TV natively play some of the more esoteric file formats that you find online?