I have a hunch that HL2 will be released almost immediately after being announced(by gaming standards). Valve seems to be on top of things, and after the TF2 debacle, I suspect that they will want to avoid the whole "building hype, declining hype" cycle that has ruined so many games. HL2 doesn't need hype; its predecessor already has the best word-of-mouth in the games industry. In fact, I suspect the game is already completed or very, very near completion. That way, news can float around for a month, long enough to get on magazine covers and around the internet, and then the finished, glorious product can immediately be available.
Of course, it's just a hunch. And probably completely absurd.