Pre-order Left 4 Dead 2 for PC on Steam or through participating retailers and you’ll be able to wear Bill’s hat from Left 4 Dead in Team Fortress 2. The hat is available right now for all to wear, so long as you’ve pre-ordered the game.
According to the Team Fortress 2 blog, the hat came into the Team Fortress 2 universe via a science experiment:
And lest you think this is just some marketing gimmick, here’s the intricate back-story justifying it: while conducting a science experiment on parallel dimensions, Bill’s hat fell off a table into the Team Fortress universe.
Okay, so that’s not true, but it’s still neat. Also, you’ll save 10% on Left 4 Dead 2 by pre-ordering the game so there’s that too.