I know Gamefly is not too popular here, but free is free. I managed to get copies of all my D2D games before they went over to GameFly, but not too sure I want to renew our relationship...
http://www.gamefly.com/download-games/
I believe its "while supplies last", also its not a Steam version...
http://www.cheapassgamer.com/forums/showthread.php?t=331512
Yeah, I grabbed my copy today even though I've already got a physical copy lying around here somewhere.
I'm still waiting on 3 of my D2D titles to be transferred over and GF won't receive a cent from me until I get those titles back. DCS Black Shark, Sword of the Stars, and STALKER Call of Pripyat.
I don't know anything about Gamefly so I can't comment on it, but what I can say is that Bioshock is one of the best games I've ever played, and I'm not even a big fan of FPS. It's just an amazing experience from start to finish, and one of the few games I've bothered to complete.