When I flew a few weeks ago, there was a guy who had a full sized acoustic bass guitar that he carried on and put in an overhead bin (sans case). I thought that was pretty funny.
On my last trip to Hawaii, the head Flight Attendant asked me if she could see my uke. She was impressed - I don't know why, it wasn't a Moore Bettah or anything - but she made a big fuss over it and kept stored it in the Crew locker. She said it was safer and more secure there. United Airlines, BTW.