Well, I took it by the shop today and talked to the mechanics. They said that it isn't uncommon for crank bolts to come loose once you've loosened them the first time. Doesn't make a whole lot of sense to me, but they were nice about it and tightened it up for me (for free). I asked if it could have gotten messed up riding w/ it loose, but they said the splines on mine (don't know what type of cranks I have) are really hard to mess up and it should be fine. I took his word for it, and I rode it a little and it seemed fine, so I'm happy. I guess time will tell if I've been screwed though... Thanks everyone for the help.
-Will