Well, it's not Half-Life 3, but Valve's other highly anticipated release of Free To Play, a feature-length documentary about Dota 2 players, has been dated: March 19.
Free To Play documents the journey of three Dota 2 players — Danil 'Dendi' Ishutin from the Ukraine, Benedict 'Hyhy' Lim from Singapore, and Clinton 'Fear' Loomis — as they compete for a million dollar prize in the first Dota 2 International Tournament. It documents the "challenges and sacrifices required of players to compete at the highest level."
Free To Play is being produced entirely by Valve and will be available to watch for free on Steam. It's not Half-Life 3, but nothing ever is.