This Portal 2 preorders page has links to where Portal 2 can be preordered, saying: "Several outlets around the world are offering special pre-order incentives including special pricing and exclusive in-game content." This covers the U.S., U.K., and several European countries, also noting that all territories can also pre-purchase through Steam, where they are offering a 10% discount on the first-person puzzle/platformer. The U.S. channels are Amazon, where they offer a $5.00 promotional credit and GameStop where they are offering GameStop exclusive customized skins for the co-op bots Atlas and P-body. Thanks Gamer's Hell. Portal 2 is currently scheduled for release on April 21, 2011. Also, Big Download notes the game's system specs are now posted. Here are those:

PC System Requirements
OS: Windows 7 / Vista / XP / 2000
Processor: 3.0 GHz P4, Dual Core 2.0 (or higher) or AMD64X2 (or higher)
Memory: 1GB XP / 2GB Vista
Hard Disk Space: At least 7.6 GB of Space
Video: Video card must be 128 MB or more and should be a DirectX 9-compatible with support for Pixel Shader 2.0b (ATI Radeon X800 or higher / NVIDIA GeForce 7600 or higher / Intel HD Graphics 2000 or higher).
Audio: DirectX 9.0c compatible

Mac System Requirements
OS: MAC OS X 10.6.6 or higher
Processor: Intel Core Duo Processor (2GHz or better)
Memory: 2GB
Hard Disk Space: At least 7.6 GB of Space
Video: ATI Radeon 2400 or higher / NVIDIA 8600M or higher