I'm not sure some of you understand the rated battery life of an iPhone 4 (or any iPhone for that matter).
Here is the rated battery life for an iPhone 4:
Talk Time - 3G 7 hours / EDGE 14 hours
Web Browsing - 3G 6 hours / WiFi 10 hours
Video Playback - 10 hours
Audio Playback - 40 hours
Standby Time - 300 hours
Overnight, after 8 hours, you will lose 2.7% battery (8/300). If you talk for 45 minutes on 3G you'll lose 10.7% (.75/7). These are all approximate numbers, with some days getting better and some getting worse. If you do several of these operations (like talking on 3G and then surfing on 3G), you need to add the losses for each time spent divided by the rating for each operation. The bottom line is if you're around the type of battery loss rated for your phone, you're fine. I normally get from 6-7 hours web browsing on 3G and 10-12 on wifi, however, I never let the battery get to cutting off, so those are approximations, since the battery loss increases the closer you get to 0%.