The other issue is the evaporative emissions. High alcohol fuel blends have about .5-1 psi higher Reid Vapor pressure than normal gasoline depending on the alcohol concentration. Highest RVP is actually at about 10% concentration and goes down some with higher percentage blends.
That puts high alcohol fuel blends outside the current specifications for RFG. The major problem in California is that not all fuel sold in the region has ethanol blended in. If you mix a fuel that was blended with MTBE to have the proper Reid Vapor pressure with a fuel that is blended to have proper Reid Vapor pressure with ethanol as the oxygenate, you get a fuel blend with too high of an evaporation rate. That causes problems with photochemical smog from evaporative emissions, which is a big concern in Calif.
When the ban MTBE out there, if they mandate that all fuel must have some ethanol in it, than as I understand it the problem of in tank mixing of different fuels will no longer be a problem.
http://www.arb.ca.gov/fuels/gasoline/oxy/updatedwvr.pdf
http://www.nrel.gov/docs/fy03osti/32206.pdf
http://www.afdc.doe.gov/pdfs/6968.pdf
Larry