I ended up taking my bike into Village Cycle Center, where I bought it, and they checked it out and fixed it for me. They tightened the brake brackets and cleaned the disc brake pads, and now there is no more annoying squeaking, so it was one of those two things that caused it.

I also want to take this opportunity to highly recommend Village Cycle for those of you here in Chicago. I have seen some other posts on this forum dogging the store as inattentive and unknowledgeable. However, my experiences there have always been positive. They have never charged me for the few minor issues my bike has had the last six months or so since I bought it, plus I still have two free autumn tune-ups that they offer customers. Moreover, their suggestions have always been helpful and their service diagnoses always correct.