In response to ddk:

It's important to realize that hydrogen is an energy CARRIER not an energy SOURCE. A hydrogen fuel cell is basically a battery utilizing different chemistry. Currently, hydrogen is usually produced using methane, not a sustainable situation. Electrolysis (separating water using electricity) is another option but takes so much electricity it is generally considered too expensive (although it may be a first step). Another option may be using high process heat from advanced nuclear power plants or other sources to drive a thermochemical process, like the sulfur-iodine process. These methods are considerably more efficient than electrolysis but rely on very high temperature steam not currently available.

As far as greenhouse gas emissions it is very dependent on the production technique. Nuclear and renewables could provide the electricity or heat carbon free. The efficiencies of fuel cells are comparable to that of batteries available today so hydrogen is unlikely to make a significant difference in this area.