Pav usually its a fault with a microswitch under drivers or passengers seat that causes this, when the switch goes faulty the car thinks there is no seat belt plugged in therefore disables the airbag system, other things could be the detanator itself (pricey) or the contacts on the steering wheel are dirty or dry. My mate had to have this microswitch thingy under his seat changed on his 350Z last summer, that little ****** cost him £400 to sort, lets hope its a dirty steering wheel contact eh, good luck dude.