From MacNN.com: iPhone 3.0 causing unusually rapid battery drain?
I don’t know if it’s iPhone OS 3.0 causing battery drain, but I do know that GPS and 3G can really use up your battery pretty quickly. I find that I have to charge my iPhone 3GS more often than my original iPhone, but that may be due to the fact that I’m using my iPhone 3GS a lot more often for things my original iPhone couldn’t do as well or at all. 3G makes surfing usable- a lot better than surfing with EDGE. Using EDGE for anything more than checking email is painful.
If you’re using GPS a lot, you’ll definitely need a car charger. Heavy use of the GPS caused my phone to run down within a few hours. The car charger that I use is the Griffin PowerJolt Redesign Home/Car Charger for iPod and iPhone which works for the iPhone 3GS.