The replies on this thread seem to head in all sorts of directions.
"Can mobiles ignite fuel?". Absolutely. Most mobiles these days have lithium batteries - poorly-manufactured batteries can and do explode. A fault (typically a short) on a mobile can also cause a high enough temperature rise on a component to ignite flammable gasses. Generally a fault of some sort or other is required for a mobile to ignite fuel - normally-functioning mobiles will not. But how do you know your mobile does not have a fault? You don't.
Specially-manufactured intrinsically-safe mobiles do exist - but they are not cheap.
"Can mobiles cause avionics equipment problesm?" Not my area of expertise, but there does seem to be examples. Better wiring and avionics standards will help this, but will take decades to become standard.
"Are mobiles on aircraft a nuisance?" Subjective opinion - the answer seems to be 'no' if it's your mobile, 'yes' if it's anyone elses.