We keep having this discussion...
In the end, it will be impracticable to ban the use of mobile phones on aircraft, if it's not so already.
First, it doesn't happen. Lufthansa has said that there is at least one phone left on during every flight they make, and I doubt this is true for just that one carrier. I'm meticulous about turning my phone off in the departure lounge before boarding, but sometimes there's a rush or a last minute gate change or... well, twice I've got off at the other end and gone to turn my mobile on, and it had been on all the time.
Second, what's a mobile phone? I play with a lot of gadgets and see a whole lot more (tough job, etc), and recently I've seen personal organisers with GPRS, GPS, Bluetooth, 802.11b or any combination of the above. All are banned in flight, all are integrated with the main device and can be disabled with more or less fiddling with the software... but not everyone's going to know how to do this, or be fully aware of what's going on in their latest toy. Can cabin crew be expected to ensure that a device is in a safe state?
Third, the carriers themselves are moving away from the 'no transmitters' rule. Lufthansa (again!) is testing a passenger wireless network linked to the Internet, Britannia has wireless laptops for the cabin crew, and Virgin (at least) is looking at having an in-cabin mobile phone base station that will let passengers make calls via their mobiles and the satellite network.
Fourth, the future is going to be more wireless. I know of at least four new standards coming into use over the next couple of years, each with its own raison d'etre and each going into a new sort of gadget as well as the current sorts.
There are only two ways to proceed, given you can't expect passengers or cabin crew to be able to comply or enforce a 'no transmitters' rule. Either you accept it as part of the aviation environment, design and test systems and train accordingly, or you start fitting wide-band radio transmission detectors to each seat, much like smoke detectors in the lavs.
At the moment, it may be statistically sensible to keep the ban and assume that enough people will follow it to substantially reduce the chances of some nasty interaction, but I don't think it's sustainable.
R