We just returned from 11 days at Coronado Springs. They had them on the beds when we got there. They look like a regular mattress pad, soft, quiet...but they have a waterproof liner. It had a label on it, indicating that it was waterproof.
I bet all Disney resorts use them. If yours doesn't....they should at least have access to them from the resorts that use them standard (like CSR).
With all the kids that visit...Disney would be stupid NOT to protect the mattresses!