According to Environmental Energy Inc., butanol can run in unmodified gas cars. In the summer of 2005, EEI drove an unmodified '92 Buick across the US running on butanol. Here is a picture of the Buick in front of the Route 66 Museum:

Treehugger

Yummy. EEI uses a patented, two-stage process to convert biomass into butanol. They use a disciplined approach in the design of their generator so that it doesn't have to be shut down and cleaned between batches. Unike most ethanol and hydrogen schemes for addressing the liquid transportation fuels crisis, it doesn't take lots of petroleum to create and distribute butanol. As EEI puts it, the plow to tire equation is positive. Further, the stream of biomass that EEI envisions using is whey permeate waste. There is a *lot* of whey permeate waste that is not being used right now that could be turned into gas for cars. Butanol can be run through pipelines, unlike ethanol, which is too corrosive. This helps with the amount of petroleom needed for distribution, since you don't have to truck the fuel to filling stations. BP and DuPont have also started to get into butanol; however, they call it biobutanol.

References and additional information: