I've always been honest and said we would definitely be back, and I've never received a promotional offer/code. Based on some of what I've read below, maybe that's not a coincidence? Maybe I shouldn't be so sure that we're returning next time?
I've wondered about this too. We just returned from the BWI and I did the survey. I said I would definitely return. Probably should have said I probably wouldn't return and then I would receive some kind of offer to get us back.
I've spent a boatload of money at WDW and I never get a code.