You don't need to be online to play it, just to activate the game.
doesn't always work like promised though. i currently have one game (Stalker Clear Sky) which STEAM insiststs is 'unavailable' even though it's installed. Client updates also sometimes causes problems with the logon information you need stored to play offline, and you won't be able to play until you can log on again.
I don't want to exaggerate, it's not a common occurrence, but enough to be annoying when it does happen.