Strange thing: Steam updates indie games in hours (Magicka, for instance, had update every day first week, with live updates after 1 hour after forum announce), but large ones like Fallout - in days.
Steam has lots of servers around the world. It takes time to copy the files to every single one before they can throw the switch.
But on the other hand, they don't have a submission process, so PC is almost always first. Sony surprised us this time.