Unfortunately for gamers, you won't be able to play the game just yet. In order to successfully install HL2 you will need to register your copy through Steam, the system is not accepting authentication requests yet however. Here's an official update from Valve:
If you have purchased a copy of Half-Life 2, we are sorry you are still waiting to play. This is not Valve's choice. Vivendi is insisting that the game has not yet been released, and has threatened that Valve would be in violation of its contract if we activate the Half-Life 2 Steam authentication servers at this time.
Thanks for your patience and we will update you when we have more news to share.
Before rushing into conclusions I think we should give Vivendi some hours to reconsider on their strategies, after all the anticipated release just happened during a weekend. If it's the case they don't make registration available until Tuesday, this will be a very dissapointing behaviour that may push legal owners of the game to look for other illegal alternatives to activate the game, that would be such a shame.