It's not a bird, and it's not superman neither

I was flying in an aeroplane a couple of weeks ago, and had put my phone in the front pocket of the shoulder bag in which I carry my laptop and laptop accessories. I did this before boarding, and when the flight attendant reminded the passengers to turn off all their electronics devices, including cell phones, portable music players, and anything with an on/off switch, I forgot all about my phone. It was nestled close to a box of mints. About 20 seconds after take-off, mid-ascent, I did suddenly remember, and quickly and inconspicuously turned it off. The Feds didn't catch me, and my phone did not accidentally engage thrust reversal.

Then on the next flight (this was a two-legged trip) our old friend Schicksal peeked around the corner. I was seated in the aisle seat of a row with three seats on each side, no one was in the middle seat, and a young woman, probably in her teens, especially judging by the relationship problems she discussed on her phone, was sitting by the window. When it was time to turn off the electronic devices, she put hers up, as expected, but as soon as the flight attendants took their seats, she pulled it back out to listen to music on it. On the ascent I thought maybe she just didn't know, or thought it was ok to listen to music on the phone or simply forgot. On the descent, however, the flight attendant specifically told her that she would have to turn it off, but she just put it in her purse to pull it out again shortly after. And on purpose too.

So my question is, what do you do in this situation? Do you politely tell the person to turn off their device? Do you inform the flight attendant if they still keep their device on? Is the risk of leaving them on really that serious? As far as I know, the main risk is interference with the airplane communications system, which could be pretty serious depending on what exactly happens. I'd hate to make a big deal out of nothing, but leaving the phone on could be disastrous, and I don't think it's ok for one passenger to decide how much risk everyone else should take. What would you do in this situation?