The problem is an odd one: under hard braking, oil may leak onto the exhaust manifold, and if that oil drips down to the non-heat-shieled portion of the manifold, it can ignite, subsequently igniting the plastics park plug wire channel, and onward from there.
Chevrolet Luminas, Monte Carlos and Impalas and Oldsmobile Intrigues built in model years 1998-2003, Buick Regals from 1997-2003 (2000 model pictured) and Pontiac Grand Prix equipped with the 3.8L naturally aspirated V6 are the affected vehicles.
How many of those cars, some of which are over 10 years old, are still on the road, is anybody's guess, but those brought in for repairs won't likely be in the shop long. The fix is simple: remove the spark plug wire retention channel and install two new spark plug wire retainers, eliminating the risk of fire.
The first cars to get the fixes are expected to come in to dealerships in May. If you might have an affected car, check out GM's Owner Center or contact your local dealer for more information.