I'm going to be an M1 at Drexel, and I've been in the Army National Guard since 1998 as a combat engineer officer. I was almost positive that I would go into an inactive status during med school, but after getting a bill for 16,000$ yesterday

, now I am starting to consider drilling as an actual possibility.
Some things for you to consider:
1. If you are considering joining the National Guard, unless you are prior service, you will have to go through basic training/AIT before you are MOS qualified (translation: you need to get your initial training to learn how to do the duties of your branch)
2. If you are an officer, you will definitely have obligations outside of your weekend drills (sometimes during the week/etc)... I was in an 'enhanced' unit in San Diego, and I would estimate that I had about 10 hours per month (unpaid) in addition to drill weekends
3. Given the current military climate, there is a non-zero possibility that your unit may get activated. Barring that, you might have to get called up for state missions (my company was called up for the LA Riots, and was drilling during Y2K in case of a worst-case disaster scenario)
4. Still, even with all the drawbacks, it's been pretty cool being a combat engineer...Where else can you get the chance to drive around in a HMMWV or armored personnel carrier, shoot an M-16, or blow up 50 pounds of C4

and then go back to work/school on Monday?
-ttac
p.s. I am not a recruiter and you should definitely get their input on the issue...