Pin trading in Disney became popular in Disney in late 1999 when purchasing pins and trading them with cast members and guests was introduced at the start of Disney's Millenium Celebration.
Since then, entire communities of pin traders have been created.
I was introduced to pin trading in 2004 when included in my Disney vacation package was 2 lanyards. I noticed people were wearing them around their necks decked out with a variety of colorful pins.
Once I realized the pins were not expensive, I decided to jump on board and begin pin collecting.
I did some pin trading with a cast member but most of the pins I bought I kept. I bought one pin of The Haunted Mansion which glows in the dark. Another pin from the 2004 Epcot International Flower and Garden Festival smells like a flower if you scratch it (and still does 4 years later!). Since I was on my honeymoon (or Disneymoon as us Disney fanatics refer to it), I was able to find a bride and groom Mickey and Minnie and a Congratulations pin from Walt Disney World. There are pins for most rides, each park and waterpark, various holidays throughout the year and specific celebrations. There are limited edition pins also. Cast members wearing pin trading lanyards will trade pins with you however, they cannot trade their name pin or any pins that are "showcase pins."
Pin trading has become popular among adults as well as kids and is an inexpensive way to have some great souvenirs from your Disney vacation. For more information on Disney Pin Trading, visit Disney's official pin trading site at http://disney.go.com/eventservices/whatispintrading.html