Keep in mind that a different style of pre-purchase rewards is being tested with XCOM: Enemy Unknown. More reward incentives are unlocked as more people pre-purchase the game. Already unlocked are an Elite Soldier Pack for the game and a variety of Team Fortress 2 items. The next unlock will be a free copy of Civilization V!
The game is slated to be unlocked on October 9, leaving a bit over two weeks to finish unlocking those pre-purchase rewards.