Ive flown to the UK (in 1998) with my fiddle and to Boston for last years Gaelic Arts festival, it was never a problem to have it as a carry-on, although the flight to the UK it turns out that the airline
restricted the carryon size (I cant remember but it was something like 30inches= the size of their compartments) and my fiddle case was over the limit. As it happened my case fit, and no one even mentioned it.
Since then, Ive been asked only to open it when going through security at the airport but again it wasnt a problem as a carry on.
But I always loosen the strings and stuff a sock in so that it doesnt bounce around.
as far as guitars go, my guitar playing friends say that while it may have been possible years ago to take a guitar as a carry on, in most cases you have to check it in with the regular luggage - so they dont think its worth the risk, and often one can borrow a guitar wherever they go.