For the 1999-2000 entering class, average overall GPA ranged from 2.94 at Meharry Medical to 3.64 at Harvard. Most schools had averages in the 3.3-3.5 range, I would say. I hear that the GPAs go up slightly every year.
Usually, you need 1 year each of gchem, ochem, gen bio, physics, math, english. The sciences usually require lab. Some schools require additional courses, such as psychology.
You also need to take the DAT which consists of bio, gchem, ochem, math, reading, and PAT (spatial ability). Most schools for the 1999-2000 entering class had an average academic score in the 18-19 range.
That's pretty much it. Most schools want you to have had some kind of exposure to dentistry (observing, volunteering, etc), but it's not usually required.