OH YEAH! You most certainly can do this...as well as use Disney Reward $$$ for adult beverages on your ticket. Just hand the reward card along with the DDP card and tell your server what you're doing. We have done this multiple trips, including as recently as May 2010. Even though we pay off the bill in full every month, we make a point to deliberately charge almost everything we buy (even groceries and daughter's room & board at college, lol) We end up with a nice chunk of change on the rewards card. Had $160 last trip...covered a lot of tips & booze
You can even "load" your rewards card at Disney (or get a new one if you're left the old one at home

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ETA...don't think this is a tipping question. It's a using-a-Rewards-card-question