I've flown a number of times with TWO guitars in hardshell cases and it was never a problem. Both my PRS and my Gibson were allowed on-board in hardshell cases. You just have to get on the plane before early enough to put them in the overhead. Luggage and stuff will still fit on top of them (if you don't mind). I put one of the guitars in the coat-closet and the other one overhead. The flight attendants didn't mind (be polite). These flights were also domestic and international. Airlines were Delta, JAL, ANA, Northwest, and one other I can't remember.
Look up the plane model before hand to get the size of the bins. Bring the printout to the gate with you to show them it will fit. BE POLITE.
I've never been asked to check my guitars except for once. But I explained the sentimental and monetary value of a '79 Silverburst and the likely hood of it getting broken if it were checked. I was given a green light.
Hope that helps.