could it be from a wrench slipping when trying to remove pedals? i don't see too many other ways that a mechanic would mess it up during the course of doing work. was the bike built up out of the box for you, or had it been sitting around the shop for a while?
yes, bikes are sometimes shipped with frame defects, and based on your description, it seems unlikely that the damage happened in shipping. and yes, mechanics do sometimes cause damage to frames. in both cases, it should be up to the bike shop to notice these things, but i still don't think that you will be able to get much out of them.