At Disney World in Orlando there is a restaurant that overlooks the castle at Magic Kingdom. The restaurant is called California Grill and it's on the top floor of the Contemporary hotel. The trick is to work with the restaurant and time your proposal for right before the fireworks display at 9pm. The staff is used to proposals so this won't be new to them. If you ask, they'll even bring out the ring on a silver platter right as the lights dim for the fireworks display. How can you go wrong?!
Oh and if you plan ahead, you can reserve the best table in the corner window, closest to the castle.
Here is the website for the restaurant:
http://disneyworld.disney.go.com/resorts/contemporary-resort/dining/california-grill/