HL2 is officially being released tomorrow at the same time world wide.
Quite the marketing gimmick. :)
Those of you who have preloaded HL2 will be able to unlock it a the designated time.
Ripster has posted the times by GMT zones in the forums.

Today is a good day for cleaning out any spyware, defragging hard drives and updating drivers.
Once your HL2 has been activated, you can right click on HL2 in the Games menu and choose "Back Up Game Files".
Keep your "copy" safe.