VG247 are reporting that Valve have finally announced a release date for Left 4 Dead 2's Cold Stream campaign. Now I know what you're thinking, you're thinking "But Tom! Isn't that campaign out already?" To which I reply "How can I hear what you're thinking, am I telepathic now?" But also "Nope, it's just been in beta testing. For a year." Which is a fairly speedy release in Valve time.
Cold Steam has spent the last year in community testing, with Valve gathering data on how the players perform on the map and adjusting it accordingly. But it's technically never been released until now, or rather until July 24th.
Assuming other Valve projects follow this release pattern, we estimate we'll be able to review Dota 2 sometime around 2014. In fact the truth is Half Life: Episode 3 was actually finished in 2009, but Valve feel it needs another six years of testing to be sure they got it right.*
*Probably not the truth.