IMAX film (although I don't remember the statistics any more) not only runs at a higher framerate, but also runs on a completely different and exponentially better quality of film. Also, remember that many IMAX movies are 3D and therefore require two polarized projections from two seperate reels.

On a side note, pointing the projection lamps at the sky on a cloudless night makes them visable to passing weather satellites...

-Craig

(used to work in an 'arts and science' museum)