Do you have to keep Bluetooth on to scan for Bluetooth Low Energy devices? When using advertise-only non-connectable Bluetooth 4.0 Low Energy / Smart devices like heart rate monitors, do you have to keep / turn Bluetooth switch on in your iPhone / iPad? 

I believe the energy drain is negligible, but the need to and explicitly turn on BT may be really frustrating for some applications.

Thanks.
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/