Check out the military medicine forums on this site. Lots of info here on USUHS, HPSP, FAP.
Also consider the Air National Guard (ANG). I'm in what's called the Early Appointment for Physicians program. Doesn't give you as much money off the bat as USUHS or HPSP but your committment is only 4 years as a reservist after you graduate. This means that you can get a normal civilian residency/practice after school--with the caveat that you could get called up for fun in the sandbox at some point (though not during school). You also get tuition assistance during school and stipends and bonuses after you graduate. In my case it means the difference between coming out of school with $200K+ in loans vs. ~$100K.
There are lots of differing viewpoints on the military and active duty vs. reserves out there. The point that most people don't get when they are initially considering these programs is that you are signing up for military service and all of the fun that entails. I've been in the military since 1998 and I happen to like the Air Force and Air National Guard (which is different from the Army National Guard by the way)