http://businessweek.com/magazine/content/05_04/b3917097_mz018.htm
"At the January auto show in Los Angeles, U.S. car enthusiasts got their first look at the H2R, a race car from BMW."
"[the] H2R race car has an ordinary internal-combustion engine"
"[the] engine can run on either hydrogen or gasoline, so simply adding a second fuel system can create a bi-fuel car."
"The downside to burning hydrogen in a combustion engine is that it produces some pollution -- a small amount of nitrogen oxides (NOx). Fuel cells, on the other hand, spew out nothing more noxious than water. But BMW asserts that its out-the-tailpipe NOx levels will be well below even California's strict Super Ultra Low Emission Vehicle standard."
"Another consideration is that BMW bi-fuel cars will use liquid hydrogen, which must be kept very cold, below -423F. The car's onboard cryogenic system takes care of this automatically. But if the vehicle isn't started up for three or four days, says Reisinger, the liquid will begin to boil, and hydrogen gas will escape through a vent. That, however, sounds like a bigger worry than it actually is. Despite persistent myths, hydrogen is less dangerous than gasoline. It disperses quickly, so even when a container leaks explosions are next to impossible."

my hs freshman year tech teacher had a v6(or v8, i forgot) engine up on a block and taking in hydrogen. neat but loud. if the engine wasnt prior used w/ gasoline, the steam exhaust would be clean enough to drink(he told us about someone who demonstrated one like that).

Now that sounds like a good teacher.