Well, if you haven't seen the gamespot videos from E3, get them...ASAP.
Doom3, well you always looked pretty, but Half-Life 2, you are the king etc.
The clip with the walker attacking in the city, the bit with the gravity gun and the mattresses...it's all astoundingly good. The graphics are fantastic, the physics are fantastic, the AI is superb, and from what you can make out on the ropy camcorder sound, the sounds is ace.
Everyone knew Valve was working on HL2, it's just that they wouldn't confirm it I do agree though that they've kept the content under wraps very well, their security must be pretty tight.
It's nearly finished, they just couldn't say anything about it when they switched to the source engine (HL2) - expect an announcement sometime around the launchdate of HL2.