One of the big draws of the Maker Faire in Austin was the Diet Coke and Mentos demonstration. I learned a couple things about the process:
1) Apparently regular coke works almost as well, but is much stickier to clean up.
2) The soda has to be warm, so don't try it with a two-liter right out of the deli fridge.
Enjoy the video of the act below, including an explanation of the physics involved, and lots of kids getting soaked. Click on the Maker Faire tag to see other videos from the event.