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The part about no harmful emissions. A gasoline fed fuel cell will produce essentially the same emissions as an internal combustion engine using the same fuel and fitted with an efficient catalytic converter. The chemical reactions are essentially the same, the oxidation of hydrogen and carbon. The end results are water and carbon dioxide. The other pollutants associated with automobiles are related to either impurities in the fuel (chiefly sulfur) or nitrogen fixation (nitrogen oxides produced by the combination of nitrogen and oxygen at high temperatures). The process for extracting hydrogen from gasoline (or any other hydrocarbon for that matter) is known as steam reforming. Typically this takes place at high temperatures in the presence of steam. The net products are hydrogen and carbon dioxide. Another source of hydrogen from hydrocarbons is partial oxidation, this is less efficient than steam reforming but can be used. There is a combiniation approach knows as autothermal reforming that appears to be the approach used in most hydrocarbon based fuel cells in development. Reforming catalysts will require essentially sulfur free gasoline, and this will of course eliminate oxides of sulfur from the exhaust gases. Of course, if the same gasoline were burned in an internal combustion engine, there would be no sulfur oxides. Of course the sulfur must be taken out somewhere, probably at the oil refinery and this will require the use of both heat (i.e. fuel burned) and hydrogen. Eventually, this sulfur will be converted to elemental sulfur and sold, this will again require more heat (i.e. fuel burned). Each step along the way will produce some polutants, mostly nitrogen oxides but also trace amounts of sulfur oxides and unburned hydrocarbons. Parial oxidation and autothermal reforming are not as sensitive to sulfur in the feed, but this sulfur will be emitted as sulfur oxides. Since air is used in both autothermal reforming and partial oxidation, there is a potential for the formation of nitrogen oxides. Both of these also produce carbon monoxide and this must be converted in carbon dioxide in a catalytic converter (just like internal combustion engines). Direct hydrogen feed fuel cells are considerably more efficient, and eliminate many of the issues just discussed, but as you have noted, the infastructure and technology for safe efficient hydrogen fueling of transportation vehicles is still to be developed. In addition, the hydrogen has to come from somewhere and the only economical way to produce this quantity of hydrogen will be by steam reforming of hydrocarbons.
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