Starting today, the Counter-Strike: Global Offensive beta is available now. The beta is private, so you are going to need a key if you want to participate. It's not sure on how you can obtain a key, but people might be getting one in their e-mails or from participating websites. Only 10,000 lucky players will be able to get a key and play only on Dust and Dust2, currently 592 players playing on servers.
The beta is to test a limited amount of weapons, items and game modes. It also gives Valve the chance to see how well their servers will respond and tests a variety of client hardware.