The FAA claims (in a study performed "last year") that they have 75 documented cases of consumer (passenger) electronic devices interfering with on-board systems,
Love to see that report. Have searched over the years of any documented, verified, incident of interference being directly attributed to consumer electronics. Wouldn't put it past a 20W Ham rig being a problem, but the typical device, meh. Cripes, half the flights have WiFi now anyway.
That said, I don't have any problem with the crew wanting your attention and focus during takeoff and landing. I can't hear announcements with my Zune and Bose noise cancelling headphones on during a flight. I expect during takeoff and landing, I might want to. There is a reason most states ban headphones when you are driving, safety. What's wrong with saying the takeoff/landing requirement is plain old safety, not some marginally defensible interference fairytail.
Oh, and yea, I certainly don't need every other person shouting into their cell phone on my flight, VoIP or otherwise. Amazing that things folks would normally whisper to each other to help keep them private, they will shout into a cell phone in a crowd.
So leave the takeoff/landing ban on use, but be up front about why.