I'm looking to do some t-shirts and I contacted someone locally who is in a band to see who he recommends. There is someone nearby who will do shirts for $4.25 a peice. I plan on buying a bunch and selling them on eBay. If all you're looking for is a t-shirt and you don't mind shipping, you may want to go to a printing place locally. You'll get better service that way. While researching, here are some places we came up with:

www.vgkids.com (local to me)
http://www.bluecollarpress.com/
http://www.t-shirts.com/
http://www.customink.com/