How long will youi be at WDW? I would suggest calling your service provider and telling them you are going to be travelling overseas and tell them you need your phone unlocked. They will email you instructions on how to unlock your phone. Once unlocked, you can now insert any SIM card into you phone and it should work. I would call Cingular first to see if they have a short term pay as you go (warning it's not as cheap as it sounds) and find out how and where you need to go to activate your phone.
Will you be calling the Mrs on vacation that much that you would invest in a pay as you go plan for the length of your vacation? I would call you phone provider over there and inquire about the cost of phone calls made from your phone in the US. Depending on the number of times you anticipate calling each other, it may be best to just use your phone. I would look into both options! Hope this helps!