They did a show about it on mythbusters. They say it's not the phone, it's static electricity from getting into your car and back out while you are pumping. You are supposed to ground yourself before touching the pump nozzle again.
Action/Results: The boys assemble a blast chamber at Fred Stoke's Gas and Oil Memorabilia Town, and equip it with an old car seat (with fuzzy tiger print cover), a cell phone and a Leyden jar which Adam has created to hold a static charge and release a spark. To create the static charge Adam has rigged a "panty static generator" using a pair of ladies' panties and some plastic tubing. The boys fill the chamber with gas vapors and call the cell phone inside, which leads to no explosion. They attempt to ignite the gas fumes with the panty generator's static charge and still get no result. The boys retire to the lab to experiment with gas vaporous and air ratios, and different sources of ignition, during which time Adam loses quite a bit of hair from standing too close to the flame. They return to Fred Stoke's and try to blow the chamber with a higher concentration of gas in a finer spray. They end up scrapping the Leyden jar in favor of something that creates a longer spark, to which they get their "big boom". The boys conclude that cell phones generate some heat, but not enough to ignite gas vaporous. They do emphasize that a static spark will ignite gas fumes at the pumps with potentially devastating results.