Cash is bad... I plan on bringing $50 or so down for the upcoming 6 day trip.
Bring credit cards and debit cards. Getting pick-pocketed in WDW is probably up there on the probability scale with getting hit by lightning, but I suppose it could happen... more likely is to simply lose the card, or purse/wallet. Before you leave, write down the customer service phone numbers off the back of these cards, and put that piece of paper in a safe spot in the luggage. That way, you can quickly call and cancel the cards, if you lose them. Another tip, if you're REALLY paranoid, is to keep one card hidden in your luggage in your room, and carry another card with you for purchases (or, if traveling with a spouse, have them carry it) - makes it very unlikely you'll both lose a card at the same time. And even if you do lose the card, your credit card company can overnight a card to you at the hotel.
All that aside, I've never lost money or a wallet at WDW. *fingers crossed*