Nick
Ask the crew. I also have a handheld and sometimes want to use it to track my location, so I always ask the crew first. Sometimes they will say yes, sometimes no, so you should accept their answer, whatever it is.
Radeng, it's quite possible, on commuter aircraft at least, that the flight crew have their own hand-held in use. If they present that much of a safety hazard I would hope that the folks at the front would have theirs switched off too.
ab