Hi Terran!
The only Walt Disney World gift registry that I know of is through Disney Weddings and Honeymoons where you could "register" for merchandise and activities that you want do to on your honeymoon and your wedding guests could "gift" it to you buy purchasing the amount the item costs and you would receive it on a gift card. The only reason why I know this is because I used it when I was a Disney bride.
I did find a generic registry tool that I found quite interesting. You can add items from anywhere, even custom list thing, as I'd imagine you will have to do for Disney activities. There is a fee for using this service, so I don't know if it would be worth it for you. You can check it out here: http://www.simpleregistry.com/
My suggestion is make your own type of registry using a document tool like Excel or Google Documents. You can even get creative and add pictures to it. List the activities and such that you are interested in for your birthday with a suggested price. That way, your friends and family can purchase Disney gift cards for you so you can use them for your vacation. You can pick up gift cards at your local Disney Store, Walgreens, most grocery stores or they can
order Disney gift cards online and choose from tons of really cool designs, even your favorite character.
Have a magical day!
Leslie