Valve has released the details surrounding pre-ordering Portal 2. The game is available for pre-order right now on Steam (and other retailers), and if you pre-order the game they’ll knock $5 off the price, bringing it down to $45.
Different retailers have different pre-order bonuses (beyond the $5 discount). Steam, for example, will throw in a free copy of the original game with every pre-order. If you already own the game, feel free to give the game to a friend or relative. Other bonuses include skins for multi-player bots.
The game is currently scheduled for release on April 18.