Well, I think everything's going to be ok. phew! The mechanic took out the bottom bracket and regreased it, and apparently the left crankarm and pedal were a little loose too.

Didn't charge me for it, which really surprised me, given that pulling out the bb is a fair amount of labor. He said that whoever installed the bb (which was done at the same shop) had used a grease that was way too light and not appropriate for the bb, and when he regreased it he used a much heavier grease, so I shouldn't be charged for correcting their mistake... he said the creaking could have come from the insufficiently greased bb, or the somewhat loose crankarm/pedal, hard to say...

Now I just hope that I didn't unknowingly cause a lot of extra wear and tear on the bb...