I recall a study they did on Icelandic crews (flight and cabin) that showed a higher incidence.
The main problem with that, and all the similar studies, is that it is impossible to correlate flying with the higher rates. Crewmembers also have more daylight hours to be on the golf course or laying out by the hotel pool, or beach, than the average "9 to 5" type. Higher sun exposure comes from a lot of places.
I think the rate IS higher, but I also think that it is due to the total lifestyle, and not the flying per se.