Ok, time for some facts:
As Keef correctly points out, strong radio-frequency (RF) fields can produce sparks between unbonded conductors (metal objects that aren't connected together to you and me) but, we are talking about typical powers of the order of 100 watts, way out of the range of mobile phones.
I too understand that the risk was first identified at quarries in the United States.
From my days as a scientist at the Safety-in-Mines Research Establishments I recall that for any particular explosive atmosphere of fuel and air there is a minimum energy spark required before ignition can take place. We designed mining equipment so that a failure at any point in the equipment would not be able to produce a spark of sufficient energy for ignition. Such equipment is known as "Intrinsically Safe".
Now, mobile phones are not Intrinsically Safe, but it is not the fact that they have a transmitter in them that creates the greatest hazard, it is the rechargable battery. Before you get too worried about this, can I remind you that a torch, a Walkman, a Camera, etc also contain batteries, and given the quality of construction, I would think that mobile phones are much safer than cheap torches.
Putting the risk into perspective, its probably similar to that from a pair of glasses (convex lenses for long-sight). i.e. too small to justify printing posters about, but we do seem to live in a crazy world.
I would guess than clothing made of man-made fibres would be a higher risk. You can get some impressive sparks from the static charges this type of clothing generates during low humidity. Not too many posters warning about this are there?
Getting hit by another car while walking to the till is probably the greatest hazard on a forecourt.