Well, we can still come up with a T-shirt for current forum members to buy now, and create more designs as time goes on and we add mroe members (and our fanbase grows to gargantuan proportions).
But I agree - polls / votes and contests are good ways of accomplishing this. In Tech's ACM, we've had in past years a T-shirt design contest. Some of them were good, some weren't, but it wss still kinda fun to see the designs.