For example, with Ericsson P800/P900, when the unit is in in-flight mode, the power amplifier is automaticaly turned off so, it neidher recieves nor sends any info to a seving cell...it's harmless since it works only with PDA functions and it can't interfere with plane's instruments. In other case, the phone will search for network and since there is no one on 10000 meters it will eventualu, after a certain period of inactivity turn PA off itself...again no send/recieve from phone and no harm... but I really can't say what happens on land while still there are available networks and how that can have influence to airplanes instruments/gear.
I think that the rule for turning off the phone is mainly there because of peace and quiet in cabin surrounding...but that's just my oppinion.
Drive safely,
SKYLINE