"For the first time in history," writes the TF2 Team, "two separate Valve universes collide!" If you pre-order Left 4 Dead 2, you get Bill's beret from Left 4 Dead to trick out your Team Fortress 2 guys.
The deal's of course restricted to pre-orders of the PC L4D2, Steam or retail. However, if you've already preordered the game, never fear. "Just fire up TF2 and you'll find the hat in your next game," the ad states.
For the record, Valve offers this defence of shamelessly rending two continuities asunder just to make a fast buck:
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.
Plausible. The hat preorder bonus accompanies a 10 percent discount, too.
Pre-Order Giveaway Madness! [TF2 Official Blog]