I may be a bit late to the party, but I just joined Pinterest. Mostly it seems to be used for crafts, recipes, interior design, and fashion, and my friends that enjoy those things have been on it for a while (beating my early adopter techy friends at early adoption). After playing with it a bit, I realized that it might also be good for mood boards, which are something I've been wanting to try but haven't had a new, from-scratch project to try them on.
Mood boards let you pull together design elements that match a particular theme ("shiny and slick", "old fashioned type and paper", etc.) to help agree on what the theme for a site should be. Pulling similar things that you like together is exactly what pinboard is good for--you can "pin" new design elements that you find on a site, add a new picture, or repin from Pinterest's very nicely browseable interface. I could imagine having clients create a collage mood board with Pinterest, especially using the repinning feature, to pull together elements that match what's in their heads, and then working with them to create a more structured mood board based on the elements they find.