While there is no official announcement yet, except for a teaser trailer, a BioShock sequel is no secret. Details on BioShock 2 are slim but today the
first info has trickled through. The sequel will "mostly" be a prequel, and the iconic Big Daddy will feature aesthetic changes, such as a shoulder-mounted wooden cannon, smaller tank on his back, leather outfit instead of the dive gear, and a much more simplified helmet with only three forward-looking sight holes, unlike the multi-directional ones in the original.
UPDATE: Take Two let us know that the forum admin posted old info and concept art, and to disregard the info.