I have a customer that has tried and sent back a fuel cell forklift. It is impractical at this time. The truck required a trailer outside to produce / store the hydrogen, and the truck had to be refuled often since it was using a same size tank as the LPG lifts. Hydrogen doesn't liquify until around 25 degrees Kelvin, so in order to pack as much fuel as a LPG truck into the same space would require obscene cryogenics + pressure (not real practical)
so what happens is that you can only store it in gas form rather than a liquid like propane.

US Department of Energy (DOE) laying out industrial targets for fuel cell cost, durability and performance.
Stack life time : 5,000hrs, Freeze start in 30sec to get 90% power at -20 degC, Power density : 2kw net/liter and Cost : 30US$/kw by 2010.
Besides fuel cell stack, fuel storage is another issue for vehicle installation and hydrogen distribution infrastructure, which forklift industry has an advantage as stated by Kodiak.
Forklift industry of which scale is roughly one hundredth to automobile industry is sitting on "Big brother", whether we like it or not. I believe no forklift manufacturer alone can hit the DOE targets set for fuel cell stack.
Two days ago, Toyota Forklift announced their prototype, rather a concept lift truck, powered by hybrid of fuel cell and capacitor at this CeMAT. Toyota fuel cell automobiles are now released to fleet users still in a small scale, but surely commercialized. Forklift industry, I think, follows in two years time.

Fuel cells are definately in the early stages. However, I don't think the forklift industry need wait on the automobile industry to pioneer this. Cost is definitly an issue but from an infrastructure standpoint forklifts lead the way.

With a large user you have quite a captive audience. Assume 50-100 units all in 1 facility. Logistically it would be easier to access 50-100 forklifts than 50-100 automobiles. An on-site fueling station would be simple compared to fitting multiple gas stations with equipment to meet a limited demand.

Companies like Cellex and General Hydrogen have tested products in warehousing applications.

There have been already a number of prototypes. But, Modern Materials Handling magazine commented "....fuel cell technology is in early development stage" (March '05 page 66).

Commercialization in this industry entirely depends (1) infrastructure and (2) cost, and we need to wait our big brother, automobile industry who is about 100 times bigger than forklift business scale, pioneers these two issues.

2007 surely be a epoch year.