Interesting. The same thing happened at
Cal Poly, except that the shirts said "I agree with Jimmy". I wasn't aware if there were "I don't agree with Jimmy" shirts available.
Personally, I thought it was a rather clever marketing concept. I finally broke down and asked one of the wearers what the shirt meant after seeing them around campus for a week. He told me that Jimmy was a christian student, and that the idea was to have a low-key way of starting a conversation about Christianity, and oh, by the way, they were having a meeting on Thursday if I wanted to come hear more about this God thing. All in all, it was a lot less annoying than being handed a tract, or visited in my home by Mormons.
I don't know if I like it more or less now that I know it was more widespread than a single campus. But one thing is for sure: if he really wants to sell the product, God needs better marketing.