I tend to experience similar battery life with my iPhone 3GS with the original post starter.
About 5~6 hours of usage and about 16 hours of standby time.
Contrary to many information online, I tend to have all my settings on.
For example, I have my screen brightness to 100%, Fetch my emails every 15 minutes, location service on all the time. I do use Wi-Fi whenever possible, but most likely when I'm outside I use 3G for talk and data.
I even read all the newspaper articles and do necessary searches with the phone.
I actually don't complain. Just wish the battery would last longer. I know having all those settings drains battery quickly, then I thought why own a device and turn off or adjust to lower settings instead of running at full capability.
Think of it this way.
It's not a phone with a computer capability. It's a computer with a phone capability.
I know it's just some terminology word play, but I never owned a notebook computer that lasted any more than five hours. So thinking that way, puts me at ease. (I know, it's not the best advice/information, but still.)