My 530i Sport doesn't have one. Don't think any of them do - they now rely on braking individual wheels - no where near as good a solution in my experience. Even the 500BHP supercharged Alpinas don't get slippy diffs as standard.
i am probably going to get a Quaife Torque biasing diff slapped into the back of my 330d at some point.
It would certainly help the wet weather traction and stop the inside wheel spinning up, which then stops the whole car accelerating as the DSC comes in