Targeted for release via Playstation Network, Xbox Live Arcade, and Steam (for PC and Mac) in early 2012, CS: GO will expand upon the team-based action gameplay that it pioneered when it was launched exactly 12 years ago (CS beta 1, August 1999).

CS: GO features new maps, characters, and weapons and delivers updated versions of the classic CS content (de_dust, etc.). In addition, CS: GO will introduce new gameplay modes, matchmaking, leader boards, and more.

"Counter-Strike took the gaming industry by surprise when the unlikely MOD became the most played online PC action game in the world almost immediately after its release in August 1999," said Doug Lombardi at Valve. "For the past 12 years, it has continued to be one of the most-played games in the world, headline competitive gaming tournaments and selling over 25 million units worldwide across the franchise. CS: GO promises to expand on CS' award-winning gameplay and deliver it to gamers on the PC as well as the next gen consoles and the Mac."

CS: GO is being developed by Valve in cooperation with Seattle-based Hidden Path Entertainment. The title is targeted for release in early 2012 and will be playable at this year's PAX Prime and Eurogamer Expo 2011.

Source Engine Changes (CS:S, DoD:S, TF2, HL2:DM)

• Fixed a packet injection exploit in the client/server streams
• Fixed UDP logging exploit
• Added a new server convar "sv_logsecret" which emits a new S2A_LOGSTRING2 UDP packet to any UDP log targets with the secret value pre-pended

More about update: here

A Steam client update is now available. To apply the update, click the File menu inside of Steam and then select "Check for Steam Client Updates...". The specific changes include:

Steam

• Changed Steam Voice chat to use the SILK voice codec
• Fixed Mac OSX overlay occasionally disappearing after 10+ minutes in game
• In game overlay for Mac OSX now handles cursor visibility better; specifically in the Sam & Max series.
• Fixed Mac OSX overlay responding correctly when window is reactivated by mouse click.
• Maximizing the Windows client should no longer prevent the user from accessing an auto-hidden taskbar
• The overlay will now automatically return to the game when accepting a friend invite
• Improved voice ducking prevention
• Fixed position of game invites in the chat dialog
• Improved handling of last modified times for cloud files.

Steam's voice chat system now leverages the SILK audio codec, developed and used by Skype, makers of the world's most popular voice communication service. The SILK codec provides a significant quality improvement over Steam's previous voice technology, at the cost of some increase in bandwidth usage. Steam Voice used to require 15 kbps of bandwidth, whereas SILK is a dynamic bit rate protocol which varies in its use of bandwidth between 8 and 30 kbps, depending on the range of data in the voice signal and current network conditions.

Steam chat with SILK is now also automatically available for all games that take advantage of the Steamworks Voice API. Valve's own Portal 2, set to release in mid-April, uses this newly updated system to enable voice chat in its cooperative gameplay mode.

• Steam Guard, a new Steam account security feature offering two-factor authentication, is now available in beta. As a Steam account holder, you can now take advantage of this additional level of account security, further prohibiting others from gaining access to your account.
  
• As a beta participant, once you've verified your email address with Steam, Steam Guard becomes available for your use and is enabled for your Steam account by default.
  
• With Steam Guard enabled, anyone attempting to login as you from an unrecognized computer must first provide additional, one-time authorization. A special access code will be sent to your contact email address, and this code must be entered into Steam before your first login on an unfamiliar computer is complete. You will also be notified if any login attempts from computers other than those you've authorized occur. Steam Guard essentially acts as a form of "User Rights Management," where you as the user have greater control over access to your stuff.
  
• More about SteamGuard Here