Fuel cells rely on some pretty volatile chemicals. Hydrogen is flammable stuff. Butane is also a flammable material. And alkali metals can be dangerous as well. But with the right fuel-cell design, these materials can remain relatively safe. You wouldn't want to toss a fuel cell into a roaring fire, but you shouldn't have to worry about your pants igniting spontaneously due to an out-of-control chemical reaction.
Depending upon the fuel cell you've got at your disposal, you may be doing the environment a favor. Fuel cells tend to have a smaller environmental impact than traditional power-generation methods. If you get your electricity from a utility that relies primarily upon coal or other fossil fuels, switching to fuel cells could reduce your own impact on the environment. If you get your electricity from a renewable source like wind power or solar power, your impact on the environment will increase.
Fuel cell technology has the potential to revolutionize more than just mobile electronics. We could see fuel cells employed in everything from emergency power supplies to vehicles. It may not be long before we no longer think about recharging our mobile devices -- we'll think about refueling them instead.