Curiosity got the better of me and I tried out fetch email for one day and I've found the battery life was pretty much exactly the same. I'm a very heavy user doing everything from lots of browsing listening to music using the location services using Siri and using Bluetooth. I find that 3G browsing uses the most battery and you just can't get around that. Switching to fetch email is counter intuitive for me I need instant email to my 4 accounts. I really like to experiment but now I realise that there is no difference, if there was a difference its not even worth talking about.