It would be interesting to find out who has been "sponsoring" the research.
I heard a rumour a while back that mobile phone companies were promoting the "turn off" principle in the air because they couldn't triangulate your position on the ground and therefore couldn't charge you. Sounds like a load of cr@p.
I'm not a pilot so I can't vouch for the frequency (or lack thereof) of such occurrences, though they do get reported from time to time. I have certainly heard the "polling" of a mobile on the air from time to time but have never seen it affect their performance
I would guess that, on average, at least one pax leaves their mobile phone on on every flight - just forgetting to turn it off. Don't hear of too many problems out there at the moment.
Not sure exactly of the power ratings of mobile phones, but don't underestimate their power either. Remember that they have enough power to transmit a modulated voice signal (digitally here in OZ anyway) for, say 50km, which is quite a remarkable device for its size.