Fine bit of paranoia, Capt. Stable, it seems to be the up-and- coming competitive sport.
Anything with the least bit of electricity in it has the potential (yes, that IS a pun) to interfere with avionics, but the likelihood of effect grows smaller in proportion to the power used. Recently manufactured commercial electronic devices which are not transmitters by nature are tested and engineered to be laregely non-interfering. Any device designed to transmit - whatever the frequency - has a greater potential. Computing devices with external cables have relatively great potential because they can randomly synthesize virtually any frequency in the range of interest, and the cables increase the ability to project those signals at powerful levels.
Assuming any small modicum of skill, your basic MK II electronic terrorist is going to ignore the cabin advisories and will use concealment anyway, so the announcements are mostly useful for ordinary folks who get a little carried away with their toys.
My experience is that your basic handheld GPS works quite well when plastered against a window in nearly all commercial aircraft, with the accurate groundspeed info especially handy.