Threadless will select a limited number of Society6 artworks and put them on prints and T-shirts that will be sold through its new "Artist Shoppe." Some of the sales revenue goes to the original artist, who just needs to post his or her work with a free Society6 account to qualify. The artist also retains control of the rights to his or her work, and will have a shot at retail distribution channels besides the Threadless website, including Urban Outfitters.
Society6 uses a newsfeed and Digg-like group promotion to help artists get attention for their work. It's quite similar to the way Threadless decides what goes in its stock of T-shirts using public submissions and social voting, so the match seems appropriate.
The Artist Shoppe is just the first of several planned collaborations between the two companies, and you can be sure the others are coming; Threadless bought a significant stake in Society6, though they'll both continue operating independently.