You'll be able to request your Disney Rewards Card on line, and they say to allow 2-3 weeks for delivery of the card. Yes, it is like a disposable gift card, and the balance will be reduced as you spend it at Disney World. It is my understanding that it can be used any place at Disney where they accept credit cards - which is just about every place except the snack carts where they only take cash.
Here's what to do if you want to be able to request your Rewards card as soon as possible. When you receive your Disney Visa credit card, call to activate it, and then use it to charge something. (You have to use it at least once in order to receive the 50 Rewards Dollars.) Set up your account for on-line access, and keep checking it. Your rewards will show up the day they mail your first statement out, and you can request your card that day - even though you may not receive the statement for several days. That's what I did. I received my statement today, and look forward to getting my rewards card within the next two weeks.