Here is something I have found that can affect battery life in an iPhone 3G or 2G, OS 2.2. I noticed it when I showed someone live video from my my home Webcam, then shut off my iPhone and put it in my pocket. About 20 minutes later I noticed the phone was warm and battery was down quite a bit.
If you go to a Web page that has a live video feed, and just exit Safari I think some background process keeps Safari updating the screen, consuming battery power in the process.
What I do when exiting Safari is to close all but one of the Safari windows, and make sure it is not dynamic content, like a Webcam or WeatherUnderground live animated weather map.