So far I've only found a U.S. reference to Niagara Falls, New York as using ClO2 municipal treatment. I presume there are others but it's clear that it hasn't been widely adopted. In California, several very large systems have switched to chloramine, which is much more stable and persistent in the distribution system than chlorine, but presents its own set of problem byproducts, plus it's quite toxic to aquatic life. And it's green.

Cheers,

Rick