For all the people who are wanting to play Portal 2 earlier than its release date, good news. If you buy the "Potato Sack" Bundle on the steam store, every purchase made will push forward the Portal 2 release date.
The Potato Sack Bundle includes 13 well known indie games for $38.72 USD and £27.22. There games like Amnesia, BIT.TRIP BEAT, Super Meat Boy and Killing floor. You also get a potato hat in Team Fortress 2.
People who are wanting to buy Portal 2 with the Potato Sack, you can buy a combined bundle for $78.71 USD and £51.21.
You can view the progress of the sales here and a countdown to the official release of Portal 2.