*update* Quake Live entered the public beta fase 24th of February 2009.
*update* Quake Live release was 6th of August 2010.
*update* Quake Live Steam release was 17th of September 2014
It is not yet known when Quake Live will be released. The game has to go through several stages in development before it reaches that point. On January the 7th the game is still in closed beta which means you need to be invited to play. Next up is the open beta, of which the date is also unknown.
Once the game is ready, id software will officially “release” the game and reset all the player data of the beta period.
If I were to guess what date Quake Live will be released I would pick Quakecon 2009, scheduled for 13 – 16 August and heavily advertised in game.
However, Quake Live is a game that’s easily updated with content by the developers. It is id software’s intention to keep the game updated constantly with new content (maps, skins, …) even when the game has been officially released.