I had the same problem, and after it happened twice, the dealer told me the next time it happened to not shut off the car and just bring it in. So I did - they ended up replacing the air bag diagnostic sensor. You can tell the trouble code by the number of times the light blinks - i think mine was blinking 5 times then pause then 3 times - trouble code 53 - which meant the air bag diagnostic was screwy. But each time I shut off the car after it would do that the light reset. Thank goodness for Ford ESP. If there's one thing I won't screw with, it's safety stuff like the airbags.