Well, today I gave a lecture in bright lemon shorts and a t-shirt with a superman torso on it. And a pair of sandshoes. I mainly wear t-shirts to lectures unless I have a particularly formal meeting that day, in which case I'll usually wear a suit and tie t-shirt
Edit: The t-shirt is for the informal conference occasions - the bits that occur after we've all been sitting around trying not to fall asleep listening to someone drone on about lattice methods or Markov random fields and the like. Us mathematicians and statisticians really like to let (what's left of our) hair down at these things. I'm usually the coolest one there of course, what with having pi tattooed on my wrist. Hard to compete with that level of coolness.
James.
Mathematicians are surely a breed of their own, not to name the crypto analysts, everyone talking with those guys walk away with their mouth open and wide open 1000 mile stare eyes.