According to Exxon Mobil, “today’s gasoline must… contain required government-mandated special components, like oxygenates (alcohols or ethers).” And according to the U.S. Geological Survey, fuel oxygenates include ethanol, along with suspect alcohols and ethers. Even though additives constitute a small portion of the gas or fuel, the use of an ethanol additive may have widespread implications for travel, people in heavily populated areas, and for those with gas heating and gas stoves. As reported earlier from a Consumer Reports article, forty percent of gasoline in the US contains some ethanol.
UPDATE: Minnesota law currently requires such a significant amount of oxygenates for the Twin Cities during the winter months that "almost all of the gasoline sold in the state is blended with 10% ethanol (E10)." Also, another Minnesota law will mandate an E20 gasoline (20% ethanol) by 2013 unless 20% of Minnesota's fuel comes from renewable resources by 2010 or unless the state does not receive permission from the US government for the use of E20 gasoline. (Minnesota House of Representatives)
For California, ethanol is the only state-approved oxygenate. Most of the gasoline in California is six percent ethanol. (California Energy Commission)
Thanks to ckpdf and kebg11 for the information on Minnesota and California.