Me and Charlotte just did 4 flights with United (of United Breaks Guitars video fame) carrying a guitar, dobro, fiddle and mandolin onto the plane and it was fine!
The third instalment of the "United Breaks Guitars" series will apparently be a bluegrass tune with Jerry Douglas on dobro. The video has just been filmed so should be up on YouTube in the coming weeks.
Sadly these piccies were taken prior to 9/11. No one gets in the cockpit other than operating crew in uniform these days. Nowadays the flightdeck door is locked against all comers and even the cabin crew have to ring through on the interphone before bringing up meals or a cuppa! All a bit sad but a sign of the times. The third photo looks like the flight engineer is holding a mike up - did this get broadcast over the cabin address system on the plane?
No, worse.... That's what I wanted to ask him when we finshed the tune (I chose 'Groundspeed' to stay on-topic). But right away there was this crackle on the speaker and then a question was heard: "What's goin'on there?' And more questions came over the radio: 'Can they play this?' 'Can they play that?' As it appeared we had been broadcasted to planes nearby (we were approaching New Found Land). We played every hour for about ten minutes anywhere in the plane (a MD-11). The guy from the agency that hired us, was following us everywhere, carrying my dobro & mandolin so we could play with some variety....
I've carried guitar, banjo and ukulele separately on flights and not had any problem (so far). It has always been decent sized planes and amenable airlines though.
My buddy was once asked by the cabin crew to play his bagpipes on a plane. They probably heard that on nearby planes without even needing the radio.