I have experienced this with the left upper side bolster on the driver's side. It is probably from brushing against it when you get in or out. The bolsters are surprisingly deep, even on the S (I wonder if it's because this is an early car). I usually don't see this kind of wear on seats because I don't wear anything abrasive....though my 17-year old Miata is exhibiting similiar wear (but that's after 149k miles).
Anyway, I showed my dealer, they took pictures and (to my surprise) are going to replace the driver's seat cover. I have to say that, since last Fall, Ford has taken care of each problem that has cropped up. Very happy thus far....