It seems so obvious, but for I've never seen anything like this! I think its very unique and a great idea. I don't think we'll do this (got enough stuff going on), but I wanted to share in case anyone was looking for an idea for favors, this is really a great idea from the offbeat bride -
Make favors that are legitimately favors you will do for your guests at some time!
http://offbeatbride.com/2010/05/iou-a-favor-wedding-favors?utm_source=feedburner&utm_medium=email&utm_campaign=Feed:+OffbeatBride+(Offbeat+Bride)