2K Games has started promoting something called "Advent," which seems to be a game about a futuristic, gated community that features surveillance for your protection, genetic mutation, and something that is definitely amiss. So, you know, BioShock. There's a site up that promotes the Advent community, with such claims as "our citizens deserve to experience everything life has to offer—free from harm and sickness. We promise nothing less than a brighter future for all." However, if you hover around the propaganda long enough, the site holds some additional secrets.
We'll probably have to wait until E3 to see what this is, but if you're a betting person, a new BioShock seems likely.