There is a lot of discussion about this subject on the newsgroup rec.music.makers.guitar.acoustic. You can try a dejaNews search for "cases".

Don't forget the other problem of theft. I have heard that one professional guitarist carries his guitar in one of those fold up things for clothes and puts his clothes in his guitar case which he checks. Then when he gets to his destination, he changes things around.

When I used to travel with photographic equipment, I put the gear in duty free bags and nobody ever questiioned my bringing them on. You would need a pretty big duty-free bag for a guitar ;)

Murray