The ground regs at my airline specifically state that SLF are not allowed to use mobiles on the tarmac (coaching operations etc..), for their own safety.
As a dispatcher I am given dispensation to use my company mobile for operational calls only - I, however, spend 8 hours a day walking around aircraft and aprons, and I'm therefore (hopefully..) less likely to walk into the prop of the Fokker 27 on the next stand through not paying enough attention to it.
As for technical issues on board the aircraft, as an ex-aircraft engineer I know that it IS possible that the phone signal can interfere with radio nav systems - this was adequately demostrated on a B737-200 some years ago, when corrosion in an aerial connector led to some very strange ILS responses in sympathy with mobile phone use. This was proven after an extensive engineering investigation, so, although PERHAPS, NORMALLY it is not really an issue, when combined with another hidden defect , it had very real safety implications. In this case, the ILS indication and F/D bars gave erroneous deviations with NO failure flags - the fault was only noticed during a visual approach when the VASI's were wildly out of kilter with the GS deviation.
The ruling refers to engine running not because there is a difference in effect of mobile phones, but because the aircraft is deemed to be able to move under it's own power at the point at which the engines are started. It doesn't really matter if the RDDMI needles are spinning in circles whilst the aircraft is connected to a tug, but it obviously does once it starts to operate the flight.