Interesting. I would think that just because the device is jailbroken, with all settings and installed apps being the same, there would be no difference in battery life. I personally have not noticed any difference in battery life of my iPhone 4 iOS 4.0.1 (jailbroken). I haven't done any testing, though. I'm pretty happy with the battery life. If I'm not really using my device, I can got a couple of days. Of course, battery life depends a great deal on what's happening with the device (what apps are running, WiFi, Bluetooth, etc.)