I suspect that MIT's security forces, even though it is a private school, hold 'peace officer' ratings and this implies that they can carry weapons at any time (though they don't usually). This holds for the small community college that I work at now as well. The officers don't carry guns as a regular part of their uniform except during registration when there are large amounts of cash on campus or when important officials visit the campus(es).
MIT Campus Police are full police officers as far as the Commonwealth of Massachusetts is concerned. They have all gone through the state police academy, and they are all cops. They have complete jurisdiction over the MIT property. And yes, they _do_ carry guns at all times (at least while in uniform). I've seen some of the MIT CP HQ, although I've not received a full tour of the facilities (maybe I will some day). Also, it is considered bad form for an MIT CP to arrest an MIT student. However I have seen them arrest people (although I've never seen a firefight ;-) -derek