Exciting post from WorldChanging.com:
In a 'Brief Communication' in the November 17 edition of Nature, Brendan Fisher and Robert Costanza from the University of Vermont show that, in the United States, up to a third of the US population lives in areas that either already have or will soon adopt policies in accord with the Kyoto protocols. Moreover, the Kyoto-friendly regions account for nearly one-half of the total US GDP -- a total economic output greater than that of Japan, currently the world's second-largest economy.
Jamais Cascio notes: The catch is that there are few mechanisms to enforce compliance at the sub-national level...
Well (as I posted in a comment over there) there's purchasing power, bond-issuing authority, permitting preferences, public education, and so much more. Half the US GDP represents a fair bit of leverage, even without national policy.
Cities can lead the change.