I think we all know by now that the battery life on the iPhone isn't spectacular. There are small tweaks that you can make that will prolong the charge a bit ... like turning of wi-fi when you don't need it. Google how to extend battery life or something since I'm not writing this to explain how. What I would like to point out is that there are some great little USB external batteries/chargers that can really help out if you are an iPhone junky. Seriously ... I have a buddy that is constantly stroking his iPhone like it is an effing magic lamp. Anyways, I recently made a purchase of one of these USB devices. Works fantastic and it is small. Check out the Lenmar PPU2100B.
It will rape you in the ol' wallet to the tune of $49 ... but it is sweet. You can get 5 iPhone charges out of it per single charge. You can also use it to supplement your usage of the iPhone. You should be able to get about 50hrs of video playback from the iPhone by using this little guy. Who the hell does that anyways? I keep one of these little bastards in my gear bag at all times. I'm a huge fan of Lenmar batteries. I think they are super sweet. I use them in my digital cameras and video cameras with great results.