The ethanol story isn't yet finished. Corn is used because it stores huge amounts of sugar in the form of sugar that converts into starch. The starch can easily be converted back to sugar and that is used in fermentation to produce ethanol.
However, substances such as cellulose are also starches (starch is a polymer of sugars) but are hard to break down to sugar so they can be fermented.
There's a lot of research (gov't sponsored) to find ways to economically hydrolyze cellulose into substituent sugars so we can get ethanol from them. I think some of the work is promising - it's not something I follow too closely but it would be truly a breakthru if we could take, for example, trash trees and brush and economically derive ethanol from them. There are some grasses (I forgot which) that already are promising.
It isn't difficult to break down cellulose to its sugars, it's just that it is not economically possible to compete with corn or with oil.
