My Trek was doing the same thing yesterday. The left pedal had developed a quiet "creeeeek" that was getting progressively louder. I was going to stop at the bike shop on my way to work to have it looked at because nothing appeared to be loose, and I thought something needed some lube. Well, halfway there the left pedal and... umm, "arm" part of the crank started getting really wobbly. Me, the genius who doesn't carry tools, had to just pray that I made it to the shop before something fell off. Well, I didn't. The left pedal and crank arm fell off, and I had to flag a police officer down to give me a ride.
The shop put everything back together, torqued it down and applied loctite. They said that might work, but if it was going to break again it would probably do so in the next week. At that point they would have to warranty the bottom bracket and crank. I really don't know what the problem was. I was in a big hurry to get back to work, and didn't really have time to ask questions.