Hockey Mom: I can't really answer your question, but can give you some info on my experience down here about how it works.
They tend to use "fake VISA/Mastercard" or as they call them, "Check card" (Kevin Bacon, Charlie Sheen commercials) down here. There aren't really the same as a true debit card. With the check card, it eliminates the step where you have to punch in "from what account - chequing or savings"
If you present your "debit card" they might not accept it, because the check cards look exactly like a VISA or MC with the little symbol down in the corner. However the check card works the same as a debit card whereby funds come directly out of your bank account.
When some friends came down from Canada they had trouble using their debit cards because they needed to put in "chequing or savings" and it didn't have that option, therefore it told them "funds not available, or transaction cancelled" (or something like that - anyway they couldn't use their card)
Personally I hate the check cards because it is much easier for theft to occur. Example: In most stores down here they run your card through the machine, but instead of waiting to punch in your PIN, a receipt comes out that you sign. You don't have to punch in a PIN.
(My first experience with this was at the DS, and I stood there waiting for the key pad, the CM was waiting for me to sign.....anyway boy did I feel dumb!)
However, in grocery stores you do you own card sliding, plus PIN (nothing to sign) but this is where is does not give you the checking or savings option and therefore it won't work.
Just be aware, they refer to these check cards as debit cards but they work slightly different than in Canada.
Your card will work for cash withdrawls at most (not all) bank machines. Service charge for these are anywhere from $1 - $3US.
Hopefully someone can give you the answer as to whether your debit card will work at WDW.