Some speculate that the Passholder postcards dont have a PIN code, others claim that it does. Some DISers have had luck with booking without a PIN and others have been told you MUST have one.
Unfortunately there is no surefire way to know who Disney sends these offers to and why. I've gone 4 times in 06, once in 07, 2 times in 08 and 23 times before 07. I received my first PIN in Jan 06, the 40% off offer. I got another in Jan 07, the 40% off offer. I did not get the 40% PIN in 2008, and got it again this year. There's no rhyme or reason.