I activated my 3G iPhone at home, and let it fully charge. I then spent a full day using the phone (Texting, surfing, and phone calls) and it finally died.
Then I recharged it and have been easily able to make it threw an entire day without charging it.
I remember hearing long ago the best way to charge a battery is to fully charge it then run it dead, recharge it fully then use it like you normally would. I'm no battery expert but this seems to work for me.
Also note that I'm not a heavy user, I do a moderate amount of texting and calling. And I spend the majority of my day on wi-fi not 3G.