I am always giving this advice, because, as a cast member, my managers have told me repeatedly to NOT recommend it if anyone comes up to me-
Do NOT (yes, caps, bold, italic, underline) propose in front of a character. While it is adorable, there is (heaven forbid) the chance she may say no. And who wants a Kodak picture of your girlfriend saying "no" in front of a big, grinning character?
Besides, another thing with WDW and their characters is efficiency. Asking her to marry you in a meet and greet line slows the entire line, especially if she gets all emotional and whatnot.
If you DO insist on doing it around characters, go to a character dining location. And do not expect to have any requests to have Cinderella and Prince Charming, or Mickey and Minnie, or any combination of characters standing together while you propose. The only chance you may have is with the chipmunks.
That being said, I'd personally like to be proposed to in some bizarre way, like while we're waiting for Fantasmic! to start, or in the middle of the Haunted Mansion...then I'd have an interesting story to tell!
If you call ahead of time, there is a possibility you can get in contact with the part of the entertainment department that does performances involving Actors Equity. I've heard of Streetmosphere planting the guy who wanted to propose in a street scene, so that might work. Doubtful, though.
OOH! Here's a great idea! If she likes stuff that goes really fast, and likes a lot of action, see if you can propose right before Lights, Motors, Action! That would be a great speech. "Honey, you've taken me on the ride of my life, and it's more exciting than any chase scene in any movie. Will you be in the seat next to me on the road of life?"
Edit
Oh! Another great place that would be fun- if you can talk to any cast members in PI at the Comedy Warehouse, they may be able to get you guys seated so that you can get the phone call (if you don't know what I'm talking about, ask, and I'd be happy to explain). If you do get that chance, also see if they can tell whoever is the first person onstage that night what you're planning. Being improv actors, they will surely be able to do something for you!