The rules are simple. Randomly CafePress will post a theme or question on their Twitter account. Followers of them can then tweet back to them linking to a shirt somewhere on their site that matches the description. You can even quickly design your own shirt, and submit that. Winners will get a free t-shirt of their choice among their entire collection!
This contest is a clever way to get people to follow them on Twitter, and fill up their site with new highly targeted content. I haven’t felt the need to sign up for a Twitter account yet, but I think this will get a lot of people to join. Check out their blog post for more information.