BioShock Follow-up a Prequel?

Next Generation is reporting that 2K Marin's follow-up to BioShock will actually be a prequel and is slated for a 2009 release.
According to internet murmurings, a BioShock follow-up is currently in pre-production and set for a 2009 release, backing up Take-Two's previous comments that the title could develop into a franchise that sees a new release every two years.

Graphic design and plot are reportedly 2K's primary focuses at the moment, and the story will likely be set before the events that took place in the original game, telling of how Rapture, a 1940s underwater utopian society, fell prey to greed and vice.