The central character (The one you play) in Operation Flashpoint: Resistance wears an Operation Flashpoint shirt (With that name accross the front) for the first few missions. (You only see it in the cutscenes as you can't see his front otherwise)
How about an Anarchy Online branded T-Shirt ingame?
More seriously - more branded clothes would be cool, like the baboons and reet suggestion above, and any other in game brands. (The gun shops that have adverts around in the game etc?)