I have exactly the same thing happening to my EVOII's as well. I got a set off Ebay last summer, all of them perfect except for 1 wheel which had a slight scratch in the clearcoat. I had that wheel refinished; and then winter came. Now the 3 wheels that weren't refinished all have this erosion - predominantly around the edges but some in a few other places as well. The wheel that was redone is still perfect!
Every time I cleaned them this winter there seemed to be little patches of oxidation in places, and cleaning that off leaves a clear "scar" in the clearcoat. As I got these off Ebay, I guess I will have to get the other 3 wheels redone as well - probably no chance of MB doing something about it. My guess is that this is a design problem though.
When I had the wheel redone, I was told that these wheels have wheelpaint in the areas between the spokes only. The outside face of the spokes is not painted at all, that is merely machined. The entire wheel then gets a layer of clearcoat. On my wheels, there is no problem in the areas that have wheel paint - only the machined outside face. Slight defects or stone chips in the clearcoat exposes bare, machined alloy; roadsalt and chemistry takes care of the rest.