I have this problem as well. According to another member, this was a voluntary recall and apparently doesn't cost the dealership anything. I would think that they SHOULD do it for free if the warranty is over. I still have a few hundred miles and weeks on my warranty. I should really bring it in.
Anyway here is the info the other member posted:
Mustang TSB and Mustang Recall Resource
Recalls & TSB's >> Tsb Lookup >> 2003 >> Ford >> Mustang
2003 Ford Mustang
Bulletin #: 042421
Sequence #: 10014138
Component Name: 151300 seat belts:front:retractor
Summary: Safety belt retraction speed. *tt
2003 Tropic Green Metallic 3.8L V6: MGW Interior Billet Accessories (knobs and pony shift), Bullitt Fuel Door, Silver Bullitt Rims w/245/45-17.