Do you have to keep Bluetooth on to scan for Bluetooth Low Energy devices?
You will need to activate Bluetooth on any iOS device in order for it to interact with other Bluetooth devices, including LE ones. 

iOS7 will add a lot of nice compatibility to BT LE devices but it looks as though even with that you will still have to turn on Bluetooth.

Source: http://9to5mac.com/2013/06/13/apple-unlocks-full-notification-center-access-much-more-for-bluetooth-le-devices-in-ios-7/