Ever since upgrading my iPhone to 2.0.2 last week, I had noticed a marked drain on my battery. I don’t use my phone that much, so it surprised me that the battery was going down as fast as it was. I never let it get fully drained, but it would easily have lost its full charge over three days, which is unusual for me.
What I forgot was that after installing 2.0.2 I also setup a mail account on the phone to my company’s Exchange server. The problem was that even though I had “Push” turned off in the “Fetch New Data” section of Settings, when I added the Exchange server, it defaulted to “Push.” Once I discovered this, turned “Push” off and told it to fetch every hour, my battery life is right back where it has always been.
So, if you’re losing battery life faster than you expected, check your “Push” settings, not just in the Settings tool, but for each email account you have setup.
Joey Gibson's Blog 
Thanks for posting this info. I was having the same problem and couldn’t figure out what was going on with my phone. I followed your suggestion above to turn “Push” to off and my battery life lasts a much longer time now.
That stinks though. I have push turned on, and my phone won’t keep a charge overnight. My Blackberry would last for days. I’d rather use the iPhone, but I sure hope this isn’t a common problem since I would find that useless.