
The Petroleum Equipment Institute (PEI) has been collecting information on this subject since the mid 1990s and reports that "almost 200 incidents have been reported to PEI that appear to be related to static electrical discharge." A full report of these incidents is available on their website. One interesting thing to note is that "The accidents occurred with conventional and vacuum assist Stage II vapor recovery nozzles," so this is not something that has been solved by modern gas pumps, as many claim.
Of the 170 fires in their report, 85 occurred when the person reentered the vehicle while fueling, and 39 occurred when the person touched somewhere near the gas cap before refueling began. So, it seems that a majority of static related fires come from static build up with the car seat.
Regarding a related myth, it should be noted that PEI has not documented a single case that was caused by a cell phone.
Check out the video below to see how quickly this type of fire can begin. This video also happens to be a good example of how to handle one of these fires. Notice how she turns off the gas flow from the nozzle before pulling it out of the tank.
For more info: Petroleum Equipment Institute - Stop Static, Report on fires at fuel pumps due to static