If there's a PC on the network running iTunes, easy:

== New AppleTV Method

With the new Apple TV there is a simpler method for this: First make sure the Repeat Music setting is set in the Audio/Video Setting section

1.create a playlist

2. drop in your videos

3. command select all the video files

4. get info on the selected files

5. change the type to Music Videos

6. On the AppleTV go to Computers > %ComputerName% > Music > Music Videos

7. Play the play list or singular video you want to repeat and away it goes