The ethanol and natural gas are both jokes.
I've got a lot of GM pride but I don't think they've done a good thing investing themselves so heavily in the whole ethanol bit. Sixseas brought up a big con - fuel economy drops. And you are still going to produce carbon dioxide. Carbon dioxide, as DoctorBill mentioned earlier, is released anytime you burn an organic (carbon based) compound. Not to mention ozone. If we start producing Ozone, it wont patch up the ozone layer, it wont just rise up and be this out of the way bi-product, it will stay in the air we breathe and ozone is nasty stuff.
The problem with natural gas is infrastructure. Natural gas must be stored and dispensed under pressure - compressed. Retrofitting gas stations and fuel systems in production automobiles would cost way more than its end worth as it too will produce carbon dioxide. Yes, it can be fermented (methane), but thats even more infrastructure change and cost, and the natural reserves we have of it are finite, just like our reserves of crude oil.
And with either of these options we still deal with the short-comings of the internal combustion engine. I grew up loving machines, cars, and the internal combustion engine, but it must change. Gasoline, LNG, CNG, Ethanol - they all use the internal combustion engine which means: Oil for engine lubrication, coolant (ethylene glycol - nasty stuff), regular maintenance, inherent inefficiencies like heat, an ideal engine speed operation (camshaft design is always a compromise, even with variable valve timing).
Conversely, electric motors have 100% of their designed torque available from a stop, they produce less heat (waste), don't require oil changes or coolant flushes, or leaky gaskets, and brushless motors require almost no maintenance at all short of the occasional bearing swap.
There are just too damn many of us these days to consume things like people did fifty years ago. Electric is the answer, save two major problems: Range (battery capacity), and infrastructure (synthesizing the electricity from solar/wind/geothermal etc..).
-MechanicMatt