There's an app in the App Store called myBatteryLife which is cute. It shows you how much time you have left on your iPhone battery depending on what you do. It's missing basic standby time, variable to brightness level and it doesn't say anything about GPS but it does make it fairly obvious some basic facts.
A) You can listen to a lot of music before the battery runs out. This was NOT the case with my previous Nokia N95
B) 3G uses approximately twice as much power as Wi-Fi. (What about WPA/WEP encrypted WiFi networks?)
C) Watching a movies is half as power thirsty as surfing the web.
I listen to news podcasts and Last.fm in the morning when cycling to work so clearly I know what not to do if the charging stations are far apart.
Judging by how rounded these figure are I guess it's fairly guesstimates. I'm currently running a Powerlog and once that file's gotten fat and levelled I'll butcher it with some scripting and try to come up with something more realistic and accurate. Watch this space.