When the professor bought us t-shirts for one particular project, it felt a bit like we were being put in our places - "Hey kids!". None of us have worn them to work. Nor will we; a formal shirt looks and feels better than a t-shirt, regardless of the quality of the t-shirt.
I think that the "t-shirts stick-it-to-the-man" attitude is pretty sad. No organization ever felt threatened by a subordinate wearing a t-shirt. Try wearing a better suit than your boss, try wearing a tailored shirt or better shoes. Rather than dressing like a rebellious teenager, try dressing like his/her boss. T-shirts seem like a rather poorly-thought-through immature act of rebellion to me.
Interestingly, the older guys in the group (40+) all wear (occasionally stained) t-shirts and claim to be completely uninterested in how they look. They also spend a lot of time taking the piss out of those of us who wear smarter clothes, telling us that we don't have to wear them. They also spend quite a bit of time (somewhat) desperately trying to convince us to dress down.
The head of the group, the professor, unfailingly wears a formal shirt and sports coat, of course.