It's pretty well documented that the iPhone 3G can barely eek out a full day's use on a single charge. So much so that a cottage industry is cropping up for external battery packs.
Here's a free tip (that I previously mentioned) for an enterprising developer that would like to sell a bunch of software: build an iPhone application that toggles on and off the 3G, EDGE, Wi-Fi, Bluetooth and even the cellular radios to conserve battery.
The Pro version (US$0.99) would add a scheduling feature that enables the user turn on and off various radios at a certain schedule. The Ultimate version (US$1.99) could add location-based toggles (via GPS and/or Wi-Fi) to switch the various radios based on where you are. (Credit to Rob Parker for the location-based idea).
I'd buy that in a heartbeat.