Half-Life 2 pre-loading begins to Steam account holders today. Over the coming days, those who have selected the pre-loading option will have encrypted Half-Life 2 files delivered to their PC. No purchase is required to pre-load HL2, but is required to activate the game. Purchase options will be released soon. Then, the moment the game is made available, those who have pre-loaded and purchased Half-Life 2 via Steam will be authenticated and ready to start playing.
By taking the suspense, challenge and visceral charge of the original Half Life 1, and adding startling new realism and responsiveness, Half-Life 2 opens the door to a world where the player's presence affects everything around him, from the physical environment to the behaviors even the emotions of both friends and enemies.
News source: Steam Website