This discussion piqued my curiosity, so I dusted off my owner's guide and had a look.
Interestingly, it suggests that I replace the plugs at 60K in both Schedule A: Severe Service and Schedule B: Normal (gentle?) Service, for the Vulcan engine.
Now, my car is much older than a 2003, but I would think that the service interval would be at least that long for a newer vehicle.
Curiously, for the Essex engine (what my car has), it recommends a 30K replacement interval for both Severe and Normal Service.
My cars have been subjected to stop-and-go delivery type abuse for the past 21 years, and I haven't noticed undue wear/deposits on the plugs. I don't generally lean on my cars very hard, but constant stop/go/idling can be tough on them as well. Many times I merely check and adjust the gap and reinstall them for continued service. Some may say that is false economy; I'm tight with my $$. I have replaced wires only once... and even that turned out to be completely unnecessary.
What I do focus on, however (regarding preventive maintenance) is servicing the engine oil and tranny fluid and filters as well as flushing the PS and brake fluid. They do degrade more quickly in such a harsh environment. The coolant seems to hold up OK. As recommended by another poster, the use of good synthetic lubricants is money well spent.
Also, stuff like brakes, suspension components and particularly engine/tranny mounts are much more prone to wear/failure. But then again, I deal with that stuff only when it breaks, or indicates that it may.
Regards... Mike