No, see Duke Nukem Forever for reference.
While the answer to the question may still be "No", I'm pretty confident in saying that Valve has handled HL3 in a much different way than 3DR handled DNF. I mean, whereas Valve has been rather consistent in suggesting that there's no fire, and no smoke, DNF was a constant smoke situation. For years it was suggested, re-suggested, over and over than not only is DNF still in development, but that things were going "well" (despite it obviously never being the case). For fans of HL, you have no real reason to be overly excited about the game, if it ever materializes. DNF on the other hand, the devs kept that train rolling even when it was obvious to everyone it was on fire.
So yeah, the answer may still be "No." Could be. But I know that I don't have *any* expectations for a HL3; I have no reason to suppose it exists. Or that it's coming. So if tomorrow Valve says "Hey guys, it's been in development and is coming out in 1 month", my personal expectations won't be built up to unreasonable levels.