If you are running iOS 5 beta, then that is probably related to the Reminders.app. For some reason (probably why it's called a beta), Reminders.app uses a crapload of battery and GPS if you have a Geo-fenced reminder on there. So your best bet could be to just go in Settings -> Location Services -> and turn off permission for Reminders.app. That's what had happened to me a while back.