I've posted another little competition on our site. You get the opportunity to win a little cash and gain mega kudos from your peers - well at least if your submission wins ;)
Essentially all you need to do is take the existing threadless thematic theme and make the checkout page look orsum! But if you do more then the more chance there is of you being a winner :)
http://www.instinct.co.nz/blog/2009/10/threadless-thematic-weekend-challenge-2/
bigbadboy