It comes down to the isolation of the wiring, the thicker the copper, the thicker the shell of the cable the less risk of interference. Problem being, thicker/heavier cables would weigh more and weight is something that one likes to avoid in aircraft. Some interference can be prevented by applying a thin sheet of aluminium around the copper wires between the isolation to provide a "shield". This works well in computer network cables (CAT5/6 shielded) as well.
Also it depends on the frequency, a WiFi signal at 2.4GHz has a far lower risk of interfering with radio signals than a GSM which operates at 800~900Mhz.
There are international laws as to which device is allowed to operate at which frequency, therefore a WiFi signal should never be able to crosstalk on the GPS frequencies used for navigation.

Small electronic items without an antenna such as non-3G Kindles, cameras, calculators, electronic games and tablets without 3G/bluetooth/wifi connectivity hardly emit any form of electromagnetic interference. It would be an awesome achievement if their "interference" can be measured more than 5 inches away from the device.

To be perfectly realistic; you can not, ever, be sure that there will be no consumer electronics on board that are switched on. People leave walkmans/iPods in their bags, coat pockets, same with cell phones. Then to think of all the devices carried in hold luggage. There are plenty of devices that can't even switch off permanently (an always ready/standby mode). If you are going to be that anal you should adapt the regulations to totally ban all Li-Ion batteries, but the public will never swallow that as it would prevent you from travelling with phones & cameras.

Taking the fact that you can not avoid electromagnetic interference due to causes mentioned above, you should solve the problem in avoiding the interference, by using different cabling/isolation/frequencies,or use a form of digital modulation that hops frequency when interference is identified. This is what most airplane manufacturers have been doing for more than a decade, 1998 seems a pretty accurate guess.

In my eyes the only reason why the "electronics issue" is still such a pain is that carriers prefer to have the full attention of their passengers during take off and landing, and the safety briefing, to ensure everybody is alert in case of a fault/dangerous situation.