According to an article at IGN, at a recent financial conference, Take-Two's chairman Strauss Zelnick states that there are already plans for Bioshock 3...before there has been any info or content released for Bioshock 2.
Furthermore, Zelnick stated that the 3rd game in the series would be released alongside the movie based on the franchise directed by Pirates of the Carribean director, Gore Verbinski.
That's a long ways away. Now if I do the math right, Bioshock came out in August of 2007. So there would be about 2 years in between the original and the sequel. If we project the same amount of time in between Bioshock 2 and 3, we should expect to see some screenshots around the time of the Mayan predicted apocolypse.
I'm actually excited to hear news about the Bioshock movie because it sounds like it has got some heavy hitters backing the project. Nontheless, I'm prepared to be dissapoitned because well...I'd rather not talk about movie based games.