"Fans have been mailing us for some time now," Valve said on the TF2 blog, "asking for a contest that didn't involve modeling or level design. So now they get their wish. That's right: you'll be competing over who can describe hats the best." The full details can be found here, but essentially, you can send in a description of one of the game's hats, and Valve might use it in the game.
In addition to tales of headwear, Valve also announced a bonus for TF2 fans who buy Worms Reloaded: Holy Hand Grenade accessories and a new helmet for the Soldier. Buy Reloaded before September 2 to get access to these (purely decorative) items.