will the hydrogen fuel cell vehicle ever hit mainstream? if so, when will they become commercially available?

is a concerted effort being made to push through development of a car that emits WATER as it's waste product? Wouldn't this solve alot of problems if it were widely used? how stable is using hydrogen as a fuel source? I keep seeing glimpses of commercials and reading snipets of info on such a vehicle, but do you think it will ever become a functional reality?