well, it's a lot harder to store than a regular gallon of gasoline. To have liquid hydrogen, you need to keep it extremely cold to keep it from turning into gas.
"I'm a dog chasing cars. I wouldn't know what to do if I caught one. I just *do* things. I'm a wrench in the gears. I *hate* plans." - The Joker http://pressthenyckbutton.blogspot.com/
Hydrogen fuel cell cars do not require liquid hydrogen (or gaseous hydrogen for that matter), but it looks like this BMW does. I think in the future, we'll see cars running on methanol or ethanol using hydrogen fuel cells and internal reformers. This is however definitely a step in the right direction.
Also, liquid hydrogen, if produced en masse for a considerable percentage of the driving public, would be relatively cheap. I think it can be produced now for $2.00 or so per gallon. However, it is currently less efficient than gas, meaning you'd get less hydrogen MPG than you would with a comparable gasoline engine. The real costs come in infrastructure and the amount of time and energy it will take to set up delivery systems for hydrogen or other alternative fuels. These costs would ultimately be passed to the consumer, so the per unit energy costs would be artificially inflated for a time.
As Simo said, this is a very good step in the right direction.