While surfing the AOPA Forums, I came across a discussion which includes inputs by folks working in and around the petroleum industry.
One of the reasons why petroleum companies are adding 10% of ethanol to gasoline, making a fuel known in the industry as E10, is that they get a tax credit for adding ethanol. This reduces their cost to blend the fuel, providing them with a powerful fiscal incentive to brew E10 even if there is no legal requirement for them to do so.
However, a more pernicious impact of blending ethanol with gasoline is outlined in a post on the AOPA forums:

Ethanol has a Motor Octane Number (MON) of about 112, much higher than auto gasoline. At the 10% level of blending it boosts the AKI of the resultant blend by about 3, so if the terminal is using 87 AKI stock and adding 10% ethanol to make E10 which is the most common ethanol blend, it is about 90 AKI. Same goes for premium unleaded.
The wrinkle comes when a whole state goes E10. Then the terminals order "sub-octane" blending stock for regular which is about 84 MON, they blend the ethanol and it goes out as 87 AKI.

If this shift to sub-octane blending stock occurs at the refinery level,the refineries will stop sending out gasoline base stock at 87 AKI, and instead send it out at 84 AKI. For premium, they will stop sending out 92-93 octane AKI base stock, and instead sent out stock at 88 AKI. There is a powerful fiscal incentive for them to do this; lower-octane base stock for gasoline is significantly easier (read: cheaper) to brew.
BTW, it is important to note that ethanol is not currently blended with gasoline at the refinery because the increased water absorptions of the gasoline caused by the ethanol makes it dangerous to send it down pipelines...sort of a larger-scale version of the same issue that may affect your plane if you fill it with E10 and go fly...
You can see what is going to happen...if an airport wants to buy Mogas i.e. 91 octane unleaded gasoline, it will not be able to find any...the refineries will all be cranking out 88 octane gasoline for ethanol addition. So, aircraft owners potentially face a "double whammy"; not only is 100LL going to become much more expensive (and it may disappear if demand shrinks enough, as it already has disappeared in parts of Europe), but Mogas could also disappear, at least at 91 octane levels, because no gasoline stock will be available from the fuel suppliers except as a custom order (which would probably price it above current 100LLL wholesale pricing).
My plane's powerplant is a 160 bhp Lycoming O-320, which could be operated on 91 octane unleaded gasoline (according to the conditions of the most common autogas STC from Petersen Aviation). However that option may become moot if there is no 91 or higher octane motor gasoline available without ethanol...
UPDATE - Here is a paper written by the EAA concerning the 2007 legislation that initially mandated the addition of Ethanol to gasoline in Oregon. It does a good job of explaining aspects of the gasoline distribution networks in Oregon, and also shows that in 2007 the refineries were already sending sub-octane base stock down pipelines to distribution terminals.