The part about the lack of leaks was something I hadn't even thought about. Other than Valve, I don't know anyone who can keep things a secret for that long, and Valve has the benefit of being a relatively small company whose staff is basically a tight-knit community not unlike a middle-American small town. And even they have stuff get leaked from time to time (the entire Half-Life 2 beta, for example, or an early version of Source Filmmaker).