The arch lining on one side of my new 156GTA was very badly fitted and I'm going to ask them to re-do it under warranty at the first service. Some crap can get under there as a result.
I'm not sure how much of an issue it is, I'm prettyy sure the whole thing is glavanised anyway. It was just poor quality control in my case - it is very obvious!
Long term it could presumably rust if mud built up - personally whenever I wash any car, arch linings or not, I spray the hose up under the wheel arches on full power to shift any deposits. My first car - a mk1 Fiesta - had an unbelievable amount of soil in the wheel arches when I got it, I dug it all out, it was like a herb garden under there! And yes the wings rusted
