I was worried about gear getting stolen or lost. I took out the Dan Equipment Policy and have an inventory page I check when arriving at the location and when I return. The policy covers loss or theft while in custody of the airlines too.
I took by gear bag as one piece of checked luggage. It had wetsuit, fins, weightbelt(no weights), rashguard, parts kit & accessories, etc. My 2nd piece of checked luggage was my clothes with room for souveniers on the way home. Both bags weighed in about 45 pounds down and about 55 back.
I had a carry on which was a small rolling carryon with a digital camera, regulators, computer, logbook and I use a ffm which also fit.
It depends on the airline too. Continental (which is what I flew) allows 50 pounds domestic but 70 pounds international. Also, a neat trick to aid your carry on - wear a photographers or fishing vest. If you need to - stuff the pockets! As of now the airlines are not charging people for being overweight. Although if they read this, they might just start!