"IMAX: Super Speedway" rocks as a demo! Daylight(DTS) and Dragonheart(DTS) are great too.

the best demonstration movie is Apollo 13.
it has all of the little perks you could
ask for. the launch is the best scene.

Since I've got HEAT, I use this DVD to show off my DD5.1 sound. The shootout scene in the street (Under Fire) sounds overwhelming when played loudly, and the final cat and mouse scene at the end of the movie (Runway cat-and-mouse) with the airplanes taking off and landing gives some incredible directional sound...

As a 2nd option I also love the opera scene in The 5th Element! *smile*