I've looked into this on CustomInk, Spreadshirt, and Cafe Press. It's nice that it's a low time commitment after the initial setup. The big downsides for me were the lack of quality control and that they take a huge chunk of profit out of every sale.
For a lower-end T-shirt sold at $20 plus shipping ($25 cost to the customer) we'd see about $2-$3 each. Done ourselves we could get a high quality t-shirt shipped to the customer for around $20 and still make $4-$5 each.
Volume is what we need to make it worth the while though. Maybe our plan should be to setup a shop and track sales for a while, then look into going back to the DIY model once sales reach a certain point that they'd be worth it. My concern with this option is that I imagine most people won't be making multiple purchases, especially if their initial purchase is of a lower quality than they expect.
Well, I didn't want to suggest it, but it sounds like it's about time for each of us to get a bag of pepperoni sticks and sell them outside the entrance of Fred Meyer