Without reading your links, some questions I would have are:
- How is the drop in usage measured? Self-reporting? If so, there might be bias there.
- Is age factored out? I would think teens are more likely to flaunt those laws and are at higher risk anyways.
- In-car navigation systems and the like seem to be rising in popularity as well. Maybe there is some cancellation there?
Really, a decrease in the percentage of the rates due to cell phones would be the interesting numbers.