New legislation that allows insurance companies to monitor a drivers mileage in exchange for a better insurance rate recently pass the California Senate Appropriations Committee.
The legislation would allow insurance companies to charge a lower rate for mileage that they can verify. While privacy advocates have issues with it, it would be an optional program. The thing is, Insurance Companies need some sort of way to set rates and mileage is a major factor.
Source: http://www.insurancejournal.com/news/west/2008/08/08/92605.htm