[wilf leek]

Turk - most likely airflow is the reason. Looks like that airflow is keeping the dust off the front wheels, but plastering it all over the rear.
Now if you had only stuck with the inboard discs on the back end you wouldn't have this problem, would you?

As to whether it means you have a brake bias problem, I doubt that in itself it does. Which end locks up first if you stomp on the pedal?
Our Single Vehicle Approval tests here, which we need to pass to get a car registered, insist that the fronts lock first, and you have to lock off any brake bias adjustment to ensure that this happens.

There are also some "performance" disc pads one can purchase in the UK which go by the name of EBC - green stuff, red stuff, silly names for different grades of pad. However, the reason I mention them is that a friend of mine who put the green stuff pads on his front noticed that no brake dust accumulated on his front wheels after that (it did before) - something to do with the composition of these pads.
Gosh - is this a first? My first serious response to a post of yours? Gotta be a record.