In this interview with Next Generation Bioshock creator Ken Levine says he's up for a sequel:

"Am I interested in continuing to work in the world?" is the first question, to which Levine answers "Yes, I am." The second question, if there is a popular demand for such work, is also answered in the positive by Levine-though in truth he need not answer at all, the game's high review scores and server-breaking sales alone telling that tale.

With the scores and sales Bioshock is getting I think I'd be more surprised if there wasn't a sequel - I just hope it doesn't come out before I get a chance to play the first game.