I haven't read all of the responses, so I apologize if this has already been mentioned.
My fiance and I got engaged last May in the Magic Kingdom. He worked with a company called Gifts of a Lifetime to set up a scavenger hunt for me. He took me to the wishing well by Cinderella's castle and told me he wanted to play a "game" and that he needed me to close my eyes and count to 100. I did (even though I felt like a big dork) and when I opened my eyes, he was gone, but there was a nice lady standing there who handed me a little card.
The card said something sweet and romantic (I can't remember now) and told me to go take a ride on the carousel. The nice lady walked with me and made sure I got on the ride, and then she disappeared. When I got off the carousel, another nice lady with a little girl came up to me and handed me another card. This one told me to go ride IASW (it was closed at the time, so I just walked over towards it) and another nice lady found me and handed me a third card, which told me to go back to the wishing well.
When I got back to the wishing well, my original "fairy godmother" was waiting for me with a card that said something about believing that a dream is a wish your heart makes and that I should meet my fiance in the rose garden. When I got to the rose garden, my fiance was waiting for me with the ring in a glass slipper he had bought. It was perfect!
Jane and her "pixies" at Gift of a Lifetime really made the whole thing very special for us. They shooed other people away from the rose garden so we had it all to ourselves, and they even took pictures with our camera of the whole proposal. They turned out great and now we have pictures of my fiance on one knee proposing with the castle in the background!
The best part is that we are going to be returning to WDW in June for our honeymoon and Jane told us that if we contact her in advance, we could be "pixies" for her if she had an event booked on a day we would be there!
I highly recommend Jane and her pixies. I know my fiance used their website, but I don't have the address...someone else here may be able to help you. Or...if you have questions, feel free to PM me!