That's the word from Valve writer Chet Faliszek, who says on the Steam forums that "there is no ARG," and there are no official hints out there at all that a third Half-Life game is coming any time soon. We don't care how convincing that secret FTP that the guy on IRC told you about was: any Half-Life 3 rumors so far are completely unofficial, as great as that fan-made Portal short was.
Then again, it would be just like Valve to say that an official ARG wasn't official, wouldn't it? Maybe they're just trying to hide the real story by saying it too is a fake! Conspiracy!