In the Ryanair case (using OnAir technology that has already been used elsewhere) the chance of such interference occuring is greatly reduced (if it exists at all) as the phones need only produce minimum power output to communicate with the on board transmitter, as opposed to maximum output to communicate with a ground station.
Only those phones that are set on automatic network selection though. If a phone is set on manual, it will continue to try to find the selected network with maximum power rather than connect with the OnAir cell. Surely, the certification of OnAir took that into account.
This is just one more reason to believe that interference from mobile phones has nothing to do with the accident in question, and is probably not much more likely than an alien laser attack. So while rumours are the gist of this forum, I would suggest to stick to slightly more realistic scenarios.