As reported by x360a:
"An eagle-eyed forum-dweller here noticed a new, secret 100 point achievement showed up on the German version of BioShock a few days ago. Well, I've been keeping tabs since, and it has now shown up on the U.S. version as well.
There has been no announcement as to what the achievement is, nor if it is currently available to be achieved. There has been talks of DLC coming with new plasmids and tonics ever since the game's launch, so it might be tied into that.
Could it be for Harvesting every Little Sister? The Anti-"Little Sister Savior" perhaps? Or collecting all the new tonics/plasmids from the DLC? There's even a rumor it could be for completing the game on Hard without dying, though that seems a stretch.
Only time will tell."