Via CAG:
Not Steam version
On site too: http://www.gamefly.com/Download-BioShock/150930/
Download the Gamefly client and open it and you'll be greeted with a orange button that says "Free" at the top and you can add bioshock to your cart and check out.
(While supplies last, so hurry!)
Not Steam version
On site too: http://www.gamefly.com/Download-BioShock/150930/