I had this same issue, but I created a video of the presentation first. (You can do this on the Mac side of course, using either Keynote or a screen capture tool.)
To then play that video on a loop in the iPad, follow these steps:
1) Create a version of the video that will play on the iPad, by importing it into iTunes and using Advanced > Create iPad version.
2) Copy that new version of the video to an iPhoto album that has nothing else in it.
3) Set that iPhoto album to sync with the iPad, and ensure that you have selected that videos should sync (in addition to just photos).
4) Perform a sync, and verify that the video did indeed sync (that the iTunes conversion succeeded; it won't sync it the video isn't iPad-playable).
5) Ensure that, as a previous poster in this thread suggested, iPad Settings for Photos says to loop them (in a slideshow).
6) Open that album that has only one thing in it, and start a slideshow. Voila.
This is the combined result of my reading several forum posts on this, and trying stuff until it worked. Good luck.
Message was edited by: NathanCarter