I got one in the mail/postcard after we came back from WDW (20% off room). I also got one recently in an email that referenced the birthday I registered online thru Disney (40% off room during birthday time frame).
My theories: I think you have to have gone there and stayed w/ them, and sign up for things online, like the birthday deal. I'm also always signing up for contests, etc. thru Disney site. I wonder if they can somehow track that I spent $ in the parks and want me back...oh, and I also have requested the trip planning DVDs the last few years. They know I want to get there!!
Can't wait to see other responses!!