What crazy hypocrisy!
The Ryanair staff are particularly stringent on the use of mobile phones. I was once asked to turn mine off as I was sending a text whilst waiting on the aircraft steps at PSA. The hosty pointed out to me that "it may interfere with aircraft systems". Likewise on finals to Stansted last year a guy in the seat in front of me got a royal rocketing from the hosty for typing a text, again the same line was quoted: "it may interfere with aircraft systems". And they’re right, it may do just that.
But from 2007 it doesn't matter if it interferes with aircraft systems, because it will be a profitable interference.
At the very least GSM signals DO produce an awful sound in the crews headsets if transmitting on high power close to the flight deck, this could result in a missed, or worse still a misunderstood ATC transmission and for this reason alone mobile phones should not be used on board.