I brought my acoustic guitar, housed in it's hard case, as carry on last July (peak busy travel season too), with no issues or problems. The plane was pretty crowded and there was no room in the overheads for the guitar, but a flight attendant took it and put it in one of the -- it escapes me right now but, the pilot's cupboard? I can't think of those thing's names right now but they took it and stored it elsewhere for me, and at the end of the flight, as I exited, the same flight attendant was standing there holding my guitar for me.
Didn't have any problems with security suspicions -- I DID take out of the case all widgets like spare strings, tuners, clipping pliers etc so that all that was in there was the guitar. Actually, I had used extra space in there to pack socks, underwear, and other soft things that wouldn't upset anyone at the x-ray machine -- utilize all your packing space on your flight, even a guitar case!
It could have gone into cargo as the case was hard (I've put a hard case electric into cargo before and it came out fine) but we were already at our allowed limit with our suitcases, and had expected to simply check in the guitar at an extra charge we were prepared to pay. But we were helpfully told by Continental staff that the guitar could be considered as one of our hand luggage pieces at no extra charge or problem (?). I think.....it has become a bit of a blur in my memory now so don't quote me on that exact detail. Plus that can vary from airline to airline. But yes, the cabin staff took care of it.
