Seth Godin hit the nail on the head and hard with his post on The T-Shirt Rule. I've been braying about this for years (even here in Pow!). Along with stickers, this is today's tangible acid test of cool among Teenagers 2.0.
And jeez, you should see my t-shirt collection! Boxes full. My current fave features the Robert Crumb drawing of Boy Howdy, mascot of Creem Magazine (my rock 'n' roll bible growing up in the '70s and '80s; currently reborn on the web).
And your fave?