fwiw, I've heard pilots making announcements and while the mic was opened up they received interference from their cell phones (which were obviously turned on).

I think that interference is the main cause for concern for the FAA, although I think they've done enough tests to realize that it isn't an issue. The vital components should be shieled well enough.

Another important reason they don't want any electronics turned on during take-off / landing is so that you can quickly react in an emergency with your undivided attention given to the crew.