Just because the t-shirt is well designed doesn't mean it is ok to put my words (or tweets) on it without asking me.
Twitshirt is poop (no matter how well crafted it is), but you can opt out if you don't want someone making money off of your work (though they will give you two bits for each $20 shirt they sell... though they won't send you any money until you reach $20 in royalties, and you won't have ANY idea that this money is waiting for you unless you sign up for Twitshirt... but Twitshirt CAN sell shirts with your words on them without you signing up. Seems fair, right?) you can opt out*. I did, and you should too.
* A note about the 'Blacklist' functionality on the Twitshirt site. It doesn't offer you any feedback after you enter your Twitter username and password and click Submit. It just dumps you have to the Twitshirt homepage. Did I successfully opt out? I have no idea.
Update: I looks like the Twitshirt folks have heard the rumblings and are hard at work 'reversing the polarity,' which I can only assume they are going to make the site opt-in instead of opt-out, which really clears up all my complaints.