Back in high school, I owned a skate shop (ran it out of my house ;) with a friend who was into graphic design. He found out that his school had a silkscreen system, so we rounded up a bunch of people interested and pressed the first tshirt, which said "TRENDY" front and center.
Beyond biting social commentary, tshirts allow you to broadcast whatever you want to the people around you, (it's like internet radio without the internet or the radio ;). Cafe Press allows you to design and print your shirts for a reasonable price, the only annoyance lies in their free store restriction on the number of designs (suggestions for hacks around that would be much appreciated).
That roughly sums up why I have a Cafe Press listing. Start your own and say what you want!