Sometimes I pack non-critical gear like spare body and lenses, extra flash etc in the check-in luggage. They have never been stolen so far, but I do this only when flying between first world destinations. It is even safer on the return flight, when theft would not be as big a problem (I have company gear, you guessed right...). This keeps the main bag, at least if crushed a bit, within the carry-on size limits. Some airlines/classes still have the rule "one (or sometimes two) carry-on bags + camera, binoculars etc", so in theory you could put a flash and the longest lens on the largest body and take that as your "camera"...