I tended to think that people were serious in turning off their cell phones during the flight- but did I understand correctly that you can be on internet during the flight? I don't see how this is possible. As for having left your cell phone on by accident for the whole duration of the flight... I don't think anything really bad can happen from that (and obviously nothing did happen). I think they just tell everyone to turn them off just so that there is less chance of anything intercepting with their functions or whatever. I guess it's a precaution to be extra safe