About a month ago, my iPhone 3G (from September 2008) started displaying serious battery life problems. That is to say, I can no longer use the phone for more than 10-15 minutes before the power cuts (without warning). Afterwards, I can only turn the phone back on by plugging it in. I have since upgrade to iOS 4, but the problem persists. I also completed a full restore, but that didn't help either. Any ideas what may be causing this? Has the battery called it quits? What may have caused this? Is there any way of fixing this? Any tips/advice would be greatly appreciated!