I've done it many times, both with electrics and acoustics. what i do is i take it to the gate, and they stick a special gate check tag on it - kinda like what they do with baby strollers. I've never had a problem. You leave the guitar on the jetway and pick it up when u get back on the jetway after the flight. I've also never had strings snap or go out of tune either as a result of flying.

A hard case is a must though. Also, like the others have said, check with the airline to make sure. I almost exclusively fly America West to go back and forth from school and I've never had a problem.