You never know when a major storm will knock out your power and leave you without juice for your phone. Battery packs like the ones from Mophie will let you charge for a short time, but if you are without electricity long enough, you will have to think outside the box to power your phone.
Hammacher & Schlemmer has an interesting cell phone charger that uses a hand crank to deliver 30-second bursts of power that'll let you make an emergency call. It also has a 2,000 mAh lithium-ion battery that'll charge your phone temporarily. The US$60 device works with most phones; charging cable is not included.