From the official Athens 2004 web site comes this list of Restricted Items and Actions:
Flag poles, logos, open umbrellas in seating areas, items (T-shirts, hats, bags, etc.) with distinctive trademarks of companies that are competitive to those of the sponsors
Yes folks, wear a t-shirt with the name of a company that’s competing with one of the sponsors, and you may well be refused admission. And who are these sponsors… sorry, “partners”? There’s a handy list here just so you don’t get confused. Suffice to say, T shirts with the words “Burger King”, “Pepsi Cola”, or “Sony” won’t be welcome. Anyone fancy going wearing a McLibel t-shirt?