I am currently serving in the NJArNG as a 2nd lieutenant in the state's Medical Command. I received my direct commission in July 2005. Before that, I was a drilling enlisted member.
During my first year of medical school, and while I was enlisted, war with Iraq broke out. My unit got activated and I was scheduled to go, even though I was in medical school at the time. At the last minute I was exempted from deployment because my position was "double-slotted" (there were two people doing my job and my unit was only authorized one person), and my commanding officer knew I was in medical school and let me stay behind. I was not exempted from deployment as a direct result of my enrollment in medical school, because I was a regular drilling member of the guard.
Shortly after this, I applied to the Guard's direct commissioning program for medical students. Once accepted, I became non-deployable until after my internship and residency obligations are fulfilled. Even then, guard and reserve physicians are limited to being deployed no more than 90 days "in-theater" per fiscal year (this does not include a 2-week in-processing and a 2-week demobilization time, resulting in 120 total days of activation). Although every single physician in my current unit has served at least one tour in Iraq, they have only done one 90-day OCONUS deployment each since the war began. A few have volunteered for multiple tours, but this was on a strictly voluntary basis.
I believe the Guard has a great program. Students are now eligible for the STRAP stipend program (~$1,300/month), annual Federal Tuition Assistance ($4,500/year) and some states (such as New Jersey) even waive medical school tuition! Although it depends on your commanding officer, most are very flexible with drills and summer training. I have never been denied a request to skip a drill or make it up some other time if it conflicts with exams or rotations.
During internship/residency, all units have the authority to use your residency time in lieu of summer training and IDT's. This is called the "Flexible Drill Policy" and is utilized at my unit. The residents are big fans because they don't have to give-up their one weekend each month to come to drill, or take 2 weeks of vacation in the summer for annual training.
Of course there are obvious disadvantages, so it's not for everyone. I just wanted to clarify the deployment policy for those in the guard's medical program.