I have both a 1st Gen 4GB iPhone and a 16GB 3G iPhone. I configured them as identically as possible including all push e-mail, network settings, etc. The only difference is my 1st Gen (personal phone) has bluetooth enabled and the 3G (work phone) does not. In my office, I can stand up and see the AT&T cell tower. It is perhaps 1/2 mile away. (Unfortunately, the 3G iPhone shows only 1 bar when I am in the office, so I keep 3G turned off.) With both phones using edge only, the 3G iPhone starts the morning at 100% and is down to 20% by noon. That is with NO usage ... no calls, no e-mail reading, no web surfing, etc. It just sits in the hip holster. The 1st Gen iPhone has been used for phone calls, e-mail reading, web surfing, etc. and is at 80-90% still.
Yesterday, I went out at lunch and purchased a home/auto charge kit so that I can keep the 3G iPhone from dying before I get home from work. My wife asked me if I was going to upgrade my personal iPhone to the new 3G iPhone at some point. My answer right now is NO.