According to VG247, Mafia II developer 2K Czech is helping Rockstar Games develop Grand Theft Auto V. Specifically, the studio is working on the cutscene animations, along with some of the general gameplay. GTA V was announced late last year, and the game is expected to arrive in 2013.
2K Czech is also rumoured to be working on Mafia III, but no official details on that project have been announced.
Source: VG247