They only accept normal mail, as I know.
I also heard that if you send an empty envelope with the text "GMail stickers", you'll get some stickers xD (haven't tried tho)
Expired on the 14th of february: http://gmailblog.blogspot.com/2008/12/get-your-gmail-stickers.html
Update (2/6): If you haven't sent in your self-addressed stamped envelope yet, it's not too late. Just make sure it's postmarked by February 14th, 2009.
Wish I had known about that sooner XD