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.Checking the Steam network's status reveals a small increase in bandwidth utilization, but at the moment, Valve isn't even tapping half of Steam's aggregate bandwidth. Despite all that excess bandwidth, Steam is telling some users that its servers are currently too busy to process their pre-load requests. It looks like Valve is carefully doling out this pre-load, at least to start. Given Steam's less than smooth debut, that's probably a good idea.
Of course, just because Half-Life 2 is pre-loading now doesn't mean you have to get the game through Steam. Half-Life 2 will come in a normal retail box, although the retail package won't hit store shelves until after the game is made available for download and activation over Steam.