I have definately seen a mobile phone upset the nav equipment on a classic 737. I was called out to the flt deck years ago (when older style mobiles were around). When I got there the nav displays and ils info were going nuts. Even somewhat unrelated stuff was affected. I turned around and saw a guy speaking on his mobile in the forward galley (right above the main electronics compartment). When we asked for him to end his call and switch off the phone everything went back to normal.
It seems that newer generation aircraft are better shielded than older types. Especially if they are Fly By Wire. When I was at Airbus they said that they specifically test for these types of interferance during flight testing. Having said that, new eqipment is being designed every day that wasn't originally tested at a/c certification, that could have adverse effect.
As my old radio instructor used to say "weird sh*t happens with radio stuff".
