Let's be scientific . . .
There is, in fact, little likelyhood of mobile phones interfering with aircraft avionics. The biggest problem is disruption to the mobile cellular systems from phones that become visible (due to altitude) to multiple base stations.
The danger on the ground is essentially nil. The airwaves at any airport are saturated with relatively high-powered RF radiation which is far more powerful (thence more likely to cause fuel explosions etc) than a wimpy 500mW GSM mobile.
It's about as silly -- scientifically -- as those signs in filling stations telling you to turn off your mobile (which, incidentally, causes it to do the very thing they don't want you to do -- it transmits a 'sign off' to the network). The same Shell filling station is likely to have a 50W T-Mobile base on the forecourt in the big sign that tells you the prices -- yes, they really do that. So if it doesn't blow up the filling station, why would your mobile? It's an urban myth, not science.
Having said all of that, the world is fiull of absurd and silly rules that we all have to obey -- so I turn off my phone before engine start.