From Joan Lowy via Classical Guitar Review.

As of February 14 2014, musicians have scored a major triumph In the ongoing battle to get musical instruments aboard flights. All too often we have heard stories of woe; snapped necks, broken bows, and demolished bridges. Now with a rather definitive law passed by US congress musicains are able to fly with more confidence.

As it pertains to guitarists, the instrument will be allowed on board without any extra charge if it can be stowed safely onboard the aircraft. It will still be in your best interest to get on the aircraft early, as those overheads fill up quickly these days, however your odds of a safe flight for your precious has just been dramatically increased!

PDF of the new FAA law (USA only)

‘‘(a) IN GENERAL.—
‘‘(1) SMALL INSTRUMENTS AS CARRY-ON BAGGAGE.—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—
‘‘(A) the instrument can be stowed safely in a suitable
baggage compartment in the aircraft cabin or under a
passenger seat, in accordance with the requirements for
carriage of carry-on baggage or cargo established by the
Administrator; and
‘‘(B) there is space for such stowage at the time the
passenger boards the aircraft.
‘‘(2) LARGER INSTRUMENTS AS CARRY-ON BAGGAGE.—An air
carrier providing air transportation shall permit a passenger
to carry a musical instrument that is too large to meet the
requirements of paragraph (1) in the aircraft cabin, without
charging the passenger a fee in addition to the cost of the
additional ticket described in subparagraph (E), if—
‘‘(A) the instrument is contained in a case or covered
so as to avoid injury to other passengers;
‘‘(B) the weight of the instrument, including the case
or covering, does not exceed 165 pounds or the applicable
weight restrictions for the aircraft;
‘‘(C) the instrument can be stowed in accordance with
the requirements for carriage of carry-on baggage or cargo
established by the Administrator;
‘‘(D) neither the instrument nor the case contains any
object not otherwise permitted to be carried in an aircraft
cabin because of a law or regulation of the United States;
and
‘‘(E) the passenger wishing to carry the instrument
in the aircraft cabin has purchased an additional seat
to accommodate the instrument.
‘‘(3) LARGE INSTRUMENTS AS CHECKED BAGGAGE.—An air
carrier shall transport as baggage a musical instrument that
is the property of a passenger traveling in air transportation
that may not be carried in the aircraft cabin if—
‘‘(A) the sum of the length, width, and height measured
in inches of the outside linear dimensions of the instrument
(including the case) does not exceed 150 inches or the
applicable size restrictions for the aircraft;
‘‘(B) the weight of the instrument does not exceed
165 pounds or the applicable weight restrictions for the
aircraft; and
‘‘(C) the instrument can be stowed in accordance with
the requirements for carriage of carry-on baggage or cargo
established by the Administrator.
‘‘(b) REGULATIONS.—Not later than 2 years after the date of
enactment of this section, the Secretary shall issue final regulations
to carry out subsection (a).
‘‘(c) EFFECTIVE DATE.—The requirements of this section shall
become effective on the date of issuance of the final regulations
under subsection (b).’’.

However, also check this out