It is all down to each individual airlines SOP's. For example, on Aer Lingus you cannot use ANY device which can receive or send signals including blackberrys!!! Now on BA you can once airborne.
I worked for Qatar before coming to BA and the safety could not be more different. However (and I do not mean this in a discriminative way) the chances of the majority of passengers on a Qatar aircraft being able to understand the commands being shouted at them by a chorus of different accents and levels of english fluency by Qatar crew would negate the requirement of not being allowed wear headsets on take off and approach! LOL!
Now back to BA. We did not have the rule about headsets until about 2 years ago. It was around this time that BA introduced the noise cancelling headsets into the premium cabins and as a result the need for these particular head sets to be not worn during critical stages of flight arose.
I also would not be too worried about the passengers situational awareness when listening to IFE programmes as at the end of the day if an evacuation or such was initiated the IFE would immediately cut out, slightly different if you were listening to your own ipod.