How is a battery's capacity (mAh) compared to a fuel cell capacity?
Fuel cells are generally rated in watts of output, not mA hours.  (As brhans pointed out the capacity depends on how big your fuel tank is.)  With a limited amount of hydrogen you could generate the rated wattage for a certain amount of time, (with the number of hours equal to the amount of fuel available/fuel consumption per hour). 

For a comparason the 3.7v 1000mAh Li-po battery has an equivalent wattage output of 3.7w for 1 hour.  So if you had a 3.7w fuel cell assembly and had enough hydrogen to last for 1 hour it would be an equivalent system.