This topic has been discussed (if not 'done to death') in the past, but to save you the use of the search function, I'll repeat my couple of thoughts here:
1. At a (calculated) guess, 99% of flights carrying more than 50 pax will have at least one active cell phone on board, either through neglect or otherwise. (Sometimes the active cell phone has been in row 0
) While this certainly doesn't prove that cell phones are safe, it suggests that any threat posed is relatively slight.
2. Now that the airlines have found a way to make money from them, cell phones will be allowed to be used on board. By installing an on-board 'picocell', the phones should be able to operate at their lowest power settings (<< 2W). At some point the picocell is going to fail in flight, and then all the active phones will swiftly ramp up to their max power settings in a futile attempt to contact the next nearest base station. This will provide an interesting test of the safety of these devices, but I'm not sure I want to be on board when it happens ...