ChristiaanJ, I think it's a fairly well known fact that mobile phones DO emit radiation on frequencies different to the actual transmission/reception frequencies. All one needs to do is stand a mobile phone near to a car radio whilst it is ringing to establish that. Now a car radio, whilst not safety critical, would not be particularly succesful if it were easily affected by EMI. The average electronic ignition system in your modern car causes quite scarily high energy levels, even outside the system, and yet in general car radio sets seem to be immune to this interference. Given this fact, it is quite concerning that a mobile phone can cause audible interference.
Granted, without thoroughly analysing the systems and emissions, one cannot say what component of the system is being affected adversely. It's quite possible that the interference is occuring in the (relatively) unshielded speaker lines rather than the internals of the system itself, but how well shielded are the transmission lines for navigational equipment? I personally am in no doubt that, in a light twin at least, a mobile phone on board CAN cause erroneous or misleading indications, I've seen it with my own eyes. Whilst this is not a direct safety issue, in that the interference is sporadic and short lived, there is always the possibility that it could lead to a pilot disbelieving his instruments and becoming disorientated.
As for a commercial environment, I think that in general the ability to use mobile phones on planes would be generally unpopular. However the ability to use wireless internet access might not prove so unpopular. The fact is, however, that the people who will decide (safety legislation aside) whether such services are supplied will be the airlines. Now some may take the view that it might be unpopular to provide such a service, but I would hazard a guess that many will find it hard to turn down another possible revenue stream. So if it can't be proved to be unsafe, I think we will probably see more widespread provision of such services onboard.