Could most commercial jet engines today run on ethanol without any problems?
Although turbines could be designed to run on ethanol it's actually a lousy aviation fuel (actually it's a lousy fuel in general). Petroleum-based jet fuel is high grade kerosene which has lubricant properties, which ethanol lacks, and ethanol has different characteristics which mean you can't pour it in and use it as a replacement.

Some problems with ethanol as a fuel are:


It's a solvent which dissolves some types of seals and hoses
It evaporates very easily, and can cause vapor lock
It is hygroscopic, meaning it attracts and mixes with water very readily. Water contamination would not settle to the lowest point, instead it would be suspended with the fuel
It has significantly less stored energy per unit of weight. Jet fuel has an energy density of 46&nbsp;MJ/kg (megajoules per kilogram) and pure ethanol has 26.4&nbsp;MJ/kg; that's only 57% of the energy density of jet fuel! Even if a jet was able to burn it at its maximum efficiency it would get just over half the oomph from it.


There are bio-fuel replacements for jet (and diesel) fuels based on oilseed crops like rapeseed (canola), palm oil, etc. These are processed to remove their OH atoms using transesterification to create fuels which are almost identical chemically. Modifying jets to use ethanol would be hugely expensive if it is even possible to do, so there's no point given there's a suitable replacement. 

The point is actually moot though because crop-based biofuels are never going to replace petroleum. There simply aren't enough crops grown to feed people and engines at the same time, in fact even if we used all the worlds farmable land to grow bio-fuel crops instead of food crops we still would not have enough of it to replace fossil fuels. We have enough trouble feeding people already and the biofuels craze is not helping that. Bio-fuels based on non-food crops from marginal land may some day become economically viable and partially replace petroleum fuels.  

After some searching I found a link to an article I wrote on biofuels for AOPA UK magazine which goes into more detail on biofuel issues.