All I can tell you is that my mustang has been winter driven all but two of the
last 9 years. There was some rust that developed in the trunk floor between the
frame rails that was taken care of by the warranty coverage. There was one spot
on the pass 1/4 panel near the rocker that I nipped in the bud a couple years
ago. Pulled out the interior and treated it from both sides. But like I say, I
nipped it in the bud. The whole repair area is maybe the size of two nickels.

I am in the chicago area BTW. The usual rust on things like swaybars has
developed but I try to take care of it as well as the little spots underneath
around body plugs and such.

All and all I think the protection is as good as any other car these days.

As Brent P said, it is SALT on the roadways that causes American cars
(not to mention imported European ones) to rust.

If you live in an area that does not apply salts to the roadways, and you
store the car in a dry area, rust will be minimal... especially if the
car was built for the North American market.  

The incidence of rustout has also greatly decreased in the last decade or
so, due to alumization/galvanizing of body panels... that you dont SEE
the zinc does not mean it's not there.