This probably has nothing to do with the update you got, I remember seeing this discussion on XDA several months ago... It is in every Rezound, but it just doesn't appear until the first time you connect an HTC Charm Call Indicator accessory to the headset jack of your phone, or your phone thinks you connected one, then it will be there forever (or until a factory reset, clean flash new ROM, etc). I can't find the thread now but seems to me the general consensus was that lint, moisture, debris, or something, gets in the headset jack and and causes just enough of a short at just the right spot and makes the phone think a charm accessory was connected, thus the menu item in settings.
