I talked with an army recruiter for several hours, and here's the lowdown:
Pay - They pay all your tuition and fees no matter what medical school you go to as long as it's in the states - for all four years.
They also pay a living stipend of ~2K/month
There are also 6 weeks a year that you are payed at a 2nd LT rate - If you aren't in school during the summer you are expected to spend that time doing medical programs with the army, but they're supposed to be really cool.
Keep in mind, though, that the pay difference in the army out of the civilian world is big - HUGE in some specialties (particularly surgery). You may take a bad financial deal in the long run, but yuou'd probably be out of debt when you graduate.
Time Commitment: Your pay-back time starts when you enter your internship/residency. However, while you are in an internship/residency, your time is also accumulating, because that is considered army education. Thus, the end effect is that you will finish your residency, and then you will still pay back years after that equal to the greater of either 4 years (for medical school), or however long your residency is (Surgery - 5 years, Neurosurgery - 7 years, etc). Thus, if you did Neurosurgery, you'd do your 7 year residency, then you'd have to serve 7 more years, for a total of 14. But, if you did family practice, you'd do your 3 year residency and then you'd have an additional 4 years to pay back for school, for a total of 7 years.
There are options for doing your residency outside of the military which changes things, but they are rare, and you'd need to talk to a recruiter for more info.