That likely means that Just Cause 2 for the PC will have support for automatic updates via Steam as well as leaderboards and Steam achievements. It also means that the game will likely not be made available for download on other PC game services like Direct2Drive, GamersGate and Impulse who have decided not to offer Steamworks-supported games on their clients. Other recent Steamworks games like Napoleon: Total War, Aliens Vs Predator, Supreme Commander 2, and most notably Modern Warfare 2 are not for sale from those outlets.
In a separate news release, the publisher announced that the PC version would support Nvidia's 3D Vision glasses and monitors for 3D effects. It will also support special Nvidia CUDA developed visual effects including more realistic looking water effects.
Gallery: Just Cause 2