when it comes to the guard I think (and I could be wrong) you only get activated as a unit as where the army reserve the can activate only certain MOSs or individual soldiers. Officers do tend to get activated for training missions more than enlisted but they usually last and where from 2 to 7 days usually (this is from what I hear and not for sure.)
Make sure the college 1st program applies to officers as well, you also can be activated before you complete OCS and have to wait until you return from deployment to get your commission, this happened to a guy in one of my grad school classes. When in state OCS his unit was activated so now he is in Iraq.
In the military everything is subject to change and it is usually for the worse, when i came in the navy i was promised that I would never wind up fighting on the ground in iraq because that isnt what we do but since then we have instated the IA (individual augmentee) program, where they remove you from your unit and send you to the army for anywhere from 6 to 18 months. This is just an example of how things work. I love serving my country and I love the navy (sometimes
) but there is a lot of sacrifice involved so just be ready to do that.