derekl
... so the cellphone functionality is completely off.
True and the device's micro processor is ON. All processors emit radio frequencies. Now, it is true that a PDA's RF output is low but it is easily measurable. It is not known where you are sitting in the cabin or what aircraft equipment/cabling may be on the other side of the decorative cabin wall.
Overall, 99.999% of these devices harmless but I am happy to have mine switched off for the duration and not use flight mode, just in case mine is the remaining percentage point.