This is done on the stanchions of high-performance mountain bike suspension forks and the body shafts of rear shocks.
Pretty cool stuff. RockShox has their own nitriding machines out in Colorado; huge bastards! Fox Racing Shox also does this new one called Kashima Coat, from the Miyaki Coporation of Japan.
Fox Racing Shox is about a half hour's drive from Apple HQ. What a logical corporate connection: mountain bikes and computers.