Portal 2 is now available for worldwide pre-order on the PC platform via Steam, and via retailers for the PS3 and Xbox 360 versions. Buying the PS3 version gives you the ability of Steam play which means Steam keys for both the PC and Mac versions as well.
Portal 2 offers cross platform play and Steamworks abilities like cloud based storage of savegames only on the PS3 and Steam versions. Steam Press Release.
Special pre-order offers vary at participating outlets around the world and include a variety of dollars off and/or exclusive in-game content for those who reserve the game for purchase before it is released the week of April 18. These include $5 off the full price, exclusive in-game skins for the coop bots (Atlas and P-body), and more.
For more Portal 2 news, screenshots and other media, head over to thinkwithportals.com