Personally, my choice would be just add the ability to hold the screen lock switch for longer in order to "secure device" and "lock screen" together. That way, if I just wanted to save some power while I'm not using the phone sitting on my desk, I can do that as I do now, and if I want to put the phone in my pocket and keep it from calling China (as my last phone did - the N900 hasn't - yet), I just hold the lock switch at little longer.