Valve sends an explanation for why Half-Life 2 is not being activated at this
time in spite of the couple reports of it being available at retail already:
Half-Life 2 For Sale?
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.