There are some issues with the implementation of the ethanol mandate in Ontario. It's to do with a specification called read vapour pressure, RVP, which is a measure of the volatility of the gasoline. Mid-grade cannot be blended on-site with non-ethanolized premium without exceeding the RVP. Whether that's the reason for the shortage, I can't comment.
On June 13th, 2007. See this statement in context.