It's even easier than that with the current version of iMovie.
...again, assuming this is AppleTV you want to share to....
Under the Share menu, choose iTunes and choose the version of the movie that will work with AppleTV (I think Large or medium might be the only two).
Assuming your AppleTV streams your iTunes movie files you will have the ability to view the movie once it's done exporting to iTunes. Otherwise, you might have to sync to get the movie to show up.
I do this all the time with my FlipHD videos...generally takes less than 15 minutes from loading to viewing for short clips.