If Cell phones were actually harmful to navigation systems, then there would surely be many more disasters caused by cellphones left on. People leave them on all the time. Not to mention that the EU is allowing in-flight cellphone use.

DO all rules make sense? no. Are the reasons due to safety? no. For example, once you land, you're allowed to turn on your cell phone, but not your ipod. Yet in flight, ipod ok, phone not ok.