Hey
Depends on how big the dress is i suppose.I got married out in Vegas and my wifes dress was like a normal dress, no trailing bit at the back. so what we did was got hold of a mini suitcase that would be allowed on board the aircraft and put in the locker above. If you fold it nicley and take it out as soon as you get there it should be fine.What ever you do dont put in the the cargo hold! Heard so many nightmare stories!
Good Luck