Threadless was created by Jake Nickell and Jacob DeHart. They came up with the idea after Jake Nickell won the NMUF London tee shirt competition. It felt good to win a tee shirt competition and we thought it'd be cool to extend that to the level that Threadless has become.
The two Threadless Jakes met on dreamless.org a while back. They both are web developers who live in Chicago and love tee shirts and posters.