From the official website of the olympics 2004:
5. Prohibitions and Restrictions 5.1 The following items may not be brought into venues: […] flags of non-participating countries, flags of participating countries that are larger than approximately 2 x 1 meters, […] flag poles, signs, umbrellas, commercial items with obvious logos of competitive companies to sponsors (hats, T-shirts, bags, etc.)

It seems that someone could be kicked out of a stadion by wearing a pepsi t-shit, because Coke is one of the official sponsors.

I guess thats Coke's revange for Coke day 1998, when a 19 year old terrorist was wearing a Pepsi shirt in high school.

Links:

• More info on Coke at the olympics, german
  
• Links to the Coke day in 1998
  
• Brilliant cartoon
  
• The COke day story