They did make a zero emissions vehicle, nobody bought it. GM is also building a wide range of vehicles that meet California's SULEV and ULEV emissions standards, which on average of 50% cleaner than the average new car (which is already pretty damn clean in the emissions area).
On top of that, they are a legitimate leader in the move to E85 ethanol.
BTW, you don't need a dedicated powerplant for biodiesel, it runs any diesel engine and GM's Duramax diesel is one of the most heavily sold in the industry (I think they have shipped over 10 million of those engines). There isn't a single import full size pickup or SUV with a diesel powerplant option, but Ford/GM/Chrysler all offer great diesel options.