I would sign up for this service, on both ends... to get a t-shirt and to advertise on t-shirts (provided I could get in on a very small scale - a few t-shirts).

As others have said, whether I would wear the shirt depends on the ad. It would be great if I could see the ad before the shirt was sent.

Also, the word 'order' almost turned me off. it immediately suggests I'd have to pay.