This is the epitome of good interaction design. There are shining nuggets of work on the internet that create this rare win-win combination, but there are just far too few. reCAPTCHA is one that surfaces – I would love to hear from anyone about other projects/products along these lines. I honestly do think this is where products are headed – all products/services will have some sort of give-back. However, they probably will not be as fun as a merry-go-round.