I always expect rain at WDW, so I plan for it. I also plan on getting wet on rides and still want to carry the camera.
I've always handled the extreme wet with zip lock bags. You can get them pretty big (way bigger than you would need). I typically carry a gallon size which will hold most of my cameras by themselves, then one of the 2 or 3 gallon size, that will fit my camera bag and all.
I've also found certain camera bags hold up to some rain better than others. One of the things I really like about my Million Dollar Home series bags is their ability to keep light moisture out. They at least keep things relatively dry while you run for cover and don the zip locks.
I have not done much with shooting while raining, but I do know there are bags out there for that (probably some of the ones already posted).