OK...I'll see if I can shed some light on this whole problem since there seems to be a ton of confusion. We have not been able to isolate an engine-related problem that causes stuttering. In fact, it turns out there are a number of *different* problems that people have lumped into the heading of "Stuttering" which has made diagnosing the problem very difficult.
I've posted a number of times that if someone in the Puget Sound area has a stuttering problem, we'd love to get our hands on your machine (I should note that we did have a number of people bring there machines in and we found stuttering caused by 1. Old drivers (video / audio), 2. Spyware / Virsuses, 3. Memory related (including one person who had a gig of ram installed but had 52 MB of available RAM after Windows started (see 2 ;-)) including physical problems with installed RAM, 4. Misc. System config problems (problems occur in other apps as well).
No one at Valve is saying that people aren't having "stuttering" problems with HL2. I'm simply saying that, with millions of copies of the game out there, we've not been able to isolate and reproduce any instances of the a problem that we can attribute to a problem with the engine.
So again...if you are having a stuttering problem and live in the Seattle area (or don't mind shipping us your rig), we're happy to take a look at your system and diagnose the problem.