In my experience http://www.zazzle.com/ or http://www.cafepress.com are the best.

Since you have the traffic, and you're a hacker and can set this up, I'd suggest that you run a T-Shirt contest.

Pay $100.00 for the top design and promise to promote the designer from your home page - let your audience vote on it.

Prior example: Shoemoney is a master at getting people to design all his stuff for free eg:

http://www.shoemoney.com/2009/05/29/design-my-new-business-c...

Good examples of sites doing this:

http://store.theonion.com/category/mens-tees-hoodies,4/big/

http://www.bustedtees.com/

Even CNN played around with this model:

http://www.cnn.com/tshirt/#headlines/allshirts/2010/6/1

I actually think the CNN-type model could work well for you - let people pick their favorite saying, make the contest about building a template / format for the t-shirt