It would get annoying as hell when this thing senses similar accelerations from non-car-hitting-you movements like bending over to roll up a pant leg or changing positions. Prankster friends would have a heyday too.
Dainese has an has an impressive motorcycle airbag suit - http://www.youtube.com/watch?v=mo9Vlt5tGwY
tl;dr - That thing would go off all that time if they made it sensitive enough to work.