Many security conscious companies routinely collect cellphones and other devices during meetings etc. NSA aside things get compromised by regular malware all the time.
I've had a small thought in the past to setup a 'luxury' service to retrofit something similar on smartphones. You would still be screwed during an actual call, since the mic would have to be on.. but a kill switch would still provide a fair amount of damage control in the event of a compromise.
If you go the extra mile and implement a 'read-only' connection to software you could remove most of the hassle for users.
Imagine moving the switch to the on position also answering an incoming call if the phone is ringing. Then when you hang up the software can send a signal to move the physical switch to the off position (but make it physically impossible to move it to the on-position from software).
Complete with a tiny LED to alert the user the switch is on.