(FORTUNE Small Business) – Despite President Bush's inclusion of a $1.5 billion investment in hydrogen fuel for cars in the 2004 tax bill, widespread use is probably 50 years away because of high costs and difficulties in converting gas stations. That hasn't stopped America's entrepreneurs. Both Hydrogen Car Co. in Los Angeles and Intergalactic Hydrogen in Steamboat Springs, Colo., adapt cars, such as Hydrogen Car's Hydrogen Shelby Cobra (above), to run on the fuel. But demand is small. Conversions cost more than $11,000, exceeding fuel savings so far. —JULIA BOORSTIN