My experience is that it's the same old crapshoot it was before 9/11. Depending on how full the flight is, when you board, what mood the flight attendants are in, what mood security or the ticket agents are, and how busy the ticket counters and security are, you may or may not be able to carry the guitar on.

I flew Delta to Atlanta earlier this month. Going down, I was able to carry my guitar (in hardshell case) on board and put it in the overhead compartment. Coming back there wasn't room and I had to gate check it, but they brought it back to me at the jetway.

OTOH, I had flights last year where they made me gate check it, then sent it through baggage claim at my destination. And a couple times the ticket agent made my fiance check her guitar at the counter.

For the record, I use a standard hard case, pack a sweatshirt or light jacket around and under the neck, and can replace the guitar for a couple hundred bucks if need be (although it's also insured on my home policy).

-- Gary