Agreed with the sentiments above. It is something that I was considering, given that my girlfriend loves travelling, and I had given it some thought.

1. Keep it classy - don't do it over the PA. It is something that should be personal, and not a public spectacle.
2. Be sure that you know she's going to say yes. Otherwise, it's going to be a damn long flight.
3. Try to involve the crew - have them serve it with dessert or as a special drink. Or you can be creative and hide the ring around the seat somewhere when she goes to the washroom. Great if you are flying in business or first class with lots of storage nooks.
4. Don't do it on economy class if possible - you're not going to be able to go down on one knee to propose properly!