there is no way to play it until it's unlocked on steam. you can help release it early by playing games in the potato pack though. see which games still need players here - http://www.aperturescience.com/glados@home/
It is IMPOSSIBLE to play it before release on steam.
Valve is very clever as they use their own encryption keys to proteect games before launch..
However, you can go to http://www.aperturescience.com/glados@home/ and try to unlock the game early.