When Take Two announced that a new Xcom was being developed for PC and Xbox 360 it was said that 2K Marin, the main development studio responsible for Bioshock 2 was developing the title. Technically, that is correct but only because 2K Marin Australia (previously 2K Australia and Irrational Games, Australia) are behind the title. They are now operating under the San Diego based 2K Marin studio, and they did help out a bit with Bioshock 2.
This also explains the persistent rumour that Irrational Games (formerly known as 2K Boston) were working on the new Xcom. Perhaps one of these days the studio naming policies of Take Two will make sense and make things less confusing. But don't count on it.