I've been having this problem intermitently for the past week or so. Sometimes multiple times in a day, always fixed by rebooting from the phone options menu. Mine always shows Airplane Mode ON on the phone options menu, however, unlike real airplane mode, my Bluetooth radio stays on. If I activate airplane mode from the phone options menu myself it works fine (including turning off the bluetooth radio and reconnecting when turned off.)
Like others have said earlier, it's tough to track down what may be going on to cause it because it's usually in the case. I don't notice until I look or sometime later realize that I haven't had any email notifications.