I got the advice that a small difference in tire pressure can cause this to happen. The car had a set of Michelins on it that had fairly good tread but closer examination revealed a lot of cracks in the sidewalls.

Since I got a good set of Bridgestone Blizzaks with the car, I had them mounted and balanced the day before the lights came back on. I never trust a tire shop's pressures and always check them after I get the car home. I did that between the time the lights came on and the next time I took the car out and they went off again. The pressures were all over the map and all too high, 36-44 psi, versus Mercedes recommended 28 front and rear. Anyway, I set them all at the recommended pressure and the lights, knock on wood, have not come back on.

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire