Have you ever tried to carry an instrument onto a plane, only to be told that it has to be checked instead?
Section 403 of the Federal Aviation Administration (FAA) Modernization and Reform Act of 2012 says that “An air carrier providing air transportation shall permit a passenger to carry a violin, guitar, or other musical instrument in the aircraft cabin, without charging the passenger a fee in addition to any standard fee that carrier may require for comparable carry-on baggage…”
If you don't remember that being the case in 2012, apparently the FAA has been really, really busy doing really, really important things and the regulations didn't actually get published in the Federal Register until Jan. 5, 2015, and so it takes effect March 6, 2015.
In other words, airlines will have to treat your instrument as they would any other piece of carry-on baggage. Which means :
1) Your instrument has to fit in an overhead bin or under the seat (So most acoustic guitars may be out right away)
2) Your instrument has to comply with the carrier's general carry-on rules, so check with the airline first
3) Just like any other carry-ons, it's first come, first served, so if it's important that you take it with you on the plane, you'd better make sure you get on board sooner rather than later.
4) If it's too big to fit as a carry-on, you may be able to have it treated as "seat baggage", which means buying it its own seat on the plane. I say "may" because this is completely up to the policies of the individual airlines.
For more details, see this article from Fretboard Journal, which goes into great detail, and gives tips and tricks for making sure your instrument (or any other piece of carry-on baggage you REALLY don't want to have to check) will have a place on the plane.