Here's a fascinating chart for Earth Day, by way of the environmental organization Earth Policy Institute.
The takeaway: more than 12 million Americans now live in jurisdictions where plastic bags at grocery stores are either banned or taxed. Even more cities – including Portland, Maine – are looking into similar policies. The trend might have started in counterculture enclaves like San Francisco, but it's quickly catching on in other big cities like Seattle (as of July 2012) and Los Angeles (as of Jan. 2014).
A handful of other cities, including Washington, D.C., and Boulder, Colorado, are taking a different tack by charging new fees for every bag dispensed, and using the funds to pay for environmental programs. At a recent City Council committee meeting, several of Portland's leaders expressed interest in pursuing this approach here in Maine.