Airlines tend to be sporadic about what the will and will not allow on board, and sometimes what they say they'll allow on the phone turns out to be different at the gate. I would HOPE you could carry it on and try -- but plan to have the guitar in a luggage-compartment safe case anyway.

Just in case. I'd hate to choose between giving up my seat on the the plane to Italy and sending an instrument to the baggage handlers in a lightweight case!