All Mercedes include this warning.The owner's manual in my '99 CLK says to never use a cell phone in the car unless it is hooked up to an outside antenae. It says the result could be a electrical malfunction that could cause an accident or damage to the car, or something along that line...
Is there any truth to this??
It is basically "CYA" (in part why you can't use cellular phones on an airplane). Although unlikely, it is possible that the phone could affect your electrical system, because of its close proximity.