The problem is common with 700s, and seems more frequent with the older years. What seems to happen is the following:

1) The inside of the rockers (the area along the underside of the car) serve as drains for the cowling
2) The rockers have little weep holes to let the water drain out
3) Because there is a big gap from teh cowling vent down the sides (hand size) debris falls down into the rockers over time.
4) The debris in the rockers gums up the weep holes
5) Water pools in the rockers
6) Pooling water mixed with chips and road damage (from both normal wear and tire detritus) cause it to rust.
7) It happens on the underside first, and it isn't noticed until it has progressed substantially.

There are also two other spots to keep your eye on for this kind of wear. The first is the underside of the doors; they have teh same drainage system as teh rockers, and have a tendency to rust when clogged.

The second is the driver and passenger floorboards; the floor is actually the back of the wheel well, and kick-up from teh tires beats off the protective sealant on teh floor to expose the metal floorboards, which then rust through. The rub is that the rug inside keeps you from seeing this damage until it is so substantial that your rugs get soaked in the rain.

-rt