We have been to WDW and DL several times and finally got a postcard in 2002 for a great introductory rate at AKL. We booked a trip on that rate, and hoped they would send us more postcards when they realized that we book a trip the day the postcard hit our mailbox!
But, I've never received another one. I'm registered at disney.com, have ordered plenty of videos, etc.
Then, we went on a big family trip in Dec. 2003 and took a friend along who'd never been before. Believe it or not, HE received a postcard in the spring. He wasn't registered online at all. I made all the reservations for our hotels so the postcard was sent to my address, but with his name on it.
So, I can't figure out what the pattern is for sending them out.