The Geiger Counter on Planet Half-Life isn't archived or permalinked, but it's got a good discussion of the current plan:

Valve has announced that they will be delivering an RC (release candidate) to Vivendi around the 15th of this month. That does not mean that HL2 will be out immediately thereafter. Vivendi will still have to approve the RC, a process that could take another two or three weeks, depending on their QA department and how fit the RC is to be released. If it were rejected, Valve would have to cook up another one and submit that for approval. Once an RC is approved, we should get a "gone gold" announcement. Those of you who preloaded HL2 and intend to buy it online would *may* be allowed to purchase it shortly afterward and start playing it once you've downloaded the last few files (I'm still not certain whether Valve contractually obliged to wait until it's on the shelves - I don't have Fragmaster's psychic spy network). For those of you still intending to buy a physical copy, it would be at least another few weeks before the game hit the shelves; duplication, packaging and shipping take a while.

The "RC on September 15" information comes from this Gamespot story.