FASHION. Shirt. A cool geek shirt is essentially a shirt, except with a bit more. Effectively, take your normal stylish shirt and add a logo, words or phrase with:
I daresay that some smart, deceptively simple, amiguous logo would probably be best. For instance the slashdot (/.) logo. I'm also partial to extremely witty, semi-ambiguous phrases or words, the sort that your average joe wouldn't understand, but that might fetch a nod and wink from passing kindred, e.g. "fsck". Or maybe I'm just being a schmoe.
One definite turnoff, however, would be a mindlessly zealous t-shirt that drivels in leet speak about the superiority of one particular operating system/software/what have you over another. That said, a little bit of Microsoft bashing is ok, as long as it isn't too zealous. I hate zeal.
For example, perhaps a good Everything2 shirt would have the e2 logo on the front, and the statement "will node for food" on the back. I suppose a URL somewhere wouldn't be too bad either.
See copyleft.net for some mostly good shirts.