Threadless is one of my all-time favorite companies and I've been buying amazing shirts from them for years. I think a Threadless Loves Glitch competition would be a fantastic collaboration. The many creative players here can submit shirt designs based on the Glitch ethos, and the community/devs get to select some of the best to be made available for purchase.
I think it would be a fun way to leverage the existing Threadless framework to promote and involve the Glitch community.