I have the same problem, and while I don't have a solution, hopefully more information will be helpful. (I also just posted this in the Apple discussion forum.) Playing around trying to figure this out yesterday, my observation was that while fetch didn't work when the phone was in standby, it did seem to work if the phone created an active network connection through some other application--e.g., if I connected to a WiFi network, or if someone sent me a text message, then mail would fetch in the background as expected. But it needed something to trigger the fetch, apparently.