Dispite reports of not being aboe to get your hands on a working copy of Half-Life 2, I heard that all steam activated accounts will go live overnight (lastnight) which means that you should have a working copy as of now.

The Steam website reports:

Half-Life 2 is available now for purchase and to play. Those who pre-purchased their copy via Steam may access the game by restarting Steam, then double-clicking on the Half-Life 2 icon in their Steam Games directory. To purchase your copy via Steam, get Steam now.