This Haagen-Dazs ad ran back in 2008, but I keep talking to my friends about it and have to rummage through my file folders every time I do, so I'll just post it here. The print ad by, I believe, Goodby, had flower seeds embedded into to the linen paper and could be planted into the ground. Goes straight to the rethink and rethink print collections.
Remember how in 2006 the ice-cream maker printed flavored stamps?
Update: Here's a very similar Green Letter from Honda, a direct-mail piece that won bronze at One Show. (Thanks, anonymous commenter.)