On freshly laid motorway tarmac even at speed the braking feels reasonably smooth, it's just on rough side streets, usually at lower speeds.
It is a bit more noticeable when the car is loaded (i.e. with more passengers).
Do you find you get any feedback yourselves through the steering wheel if the motorway is flat? (I get very little myself if the road is smooth).
I did some research on it last night and it's called "bump steer". Apparently it can be minimised by changing the geometry of the track rod ends.http://forums.bimmerforums.com/forum/showthread.php?t=178221