My guess is that HL2 is more or less ready to go, and that what is holding things up is all this Steam nonsense. If there were still significant fixing to do on HL2, enough so that they could not make their release date, they would have known long before now.

The whole Steam project smells of nothing so much as a last-minute idea that grew, as software projects often grow, out of hand. It's called "creeping featurism", and it most frequently happens when someone who doesn't listen to his software engineers is calling the shots.