I have recently started to notice this problem and I have every intention of fixing it on my own. I just haven't had time to get to it. The salt will fly soon and I need to get it done before that.
I'm going to sand out the rusted areas and spray them with underbody coating. I don't have anything on the outer part of the fender. Just in the wheel well. I don't think it's going to get bad enough in time for me to get the dealership to cover it.
EDIT: I just read the warranty for my 2001: 12 years / unlimited mileage for perforated rusty sheet metal body panels. So, maybe I'll stop by a dealership.