However when somebody leaves their mobile phone on during the flight it will have no impact what so ever on the aircraft.
Think about it. Several airlines a contemplating the idea of allowing passengers to use their mobile phones during flight.
I'm an Electrical Engineer, not a pilot, and spent several years of my career on reliability matters at a major semiconductor manufacturer.
As noted by others, a mobile phone desperately trying for a link can raise transmit power to levels which drain the battery at a rate which quickly heats the phone uncomfortably hot. This will not happen with the contemplated onboard picocell arrangements. That difference reduces a safety concern. While most lithium cells are reliable even under heavy load, a few contain defects which incline them to catch fire or explode in that case. So the cells contain safety interrupts, which stop things before they become beastly--when
they work properly.
But if the cheese holes line up too well:
1. turned-on phone
2. very distant cell tower causes the phone to raise transmit power
3. lithium cell has inherent manufacturing defect so tries to blow up.
4. included safety device is also defective, so the attempt succeeds.
You now have a bit of passenger explosion/fire aboard. Still several more cheese holes to go before disaster.
I'm one who guesses the picocells will enhance safety, by greatly reducing the number of mobile phones transmitting at high power on portions of each flight. Which I suspect will outbalance any safety loss from increase interference.
With pretty much every laptop vendor having gone through lithium cell recalls induced by actual in the field fire/explosion events, the probability that both conditions 3 and 4 will obtain is higher than I'd to keep hoping we shall continue lucky to drop both 1 and 2 as barriers.
Oh, and yes, I do turn off my cell phone when instructed.