Surely it's just a myth that leaving them turned on will stuff up the plane's electrical system?
I haven't read much about it, but when I was on a Virgin Blue plane some weeks ago, they firmly made the point across that it was imperative that we do turn them off because they may interfere with the plane's technical side of things or some crap.
Surely if it was anything too major keeping them turned on, they'd actually make sure everyone locked them up in a special compartment on the plane - turned off - so we didn't have access to them mid-flight, in case someone decides it's their goal to crash the plane.
Just seems strange.