I also have an iPhone 3G and had been having problems with my battery. After a complete charge my phone was out of battery 4-5 hours later without even using it. I went to the apple store and got a replacement thinking the phone was faulty. I set up all my accounts right away and 4 hours later the battery was in red once again. The only thing I could think of was my outlook account was causing the problem, so I went ahead and turned it off. Results: My phone is fully charged and is already 4 hours since I last charged it. So the only thing I can say is: the problem is when you have an exchange account in the iphone, but my guess is the problem is only with exchange 2003 since my husband also has his outlook on the iphone but he doesnt have this problem, the only difference is he has exchange 2007. Maybe as someone mentioned, it could be a bad configuration on the server side. At least I found what the problem was and now I removed the push option and changed to manual. Lets see how that turns out.
If you are having the same problem, please share, to see if maybe the problem is with 2003.