We had an air bag light on. Searched the net and found out cars like ours had been recalled by the manufacturer - check the VOSPA site (got it fixed for nothing).
If you had of had an accident it would of been a bit more than a light on,one of the common problems you get with airbag lights is poor/broken connection to wiring under the front seats, the movement of the seat when being adjusted can cause it. NOt saying it is, but worth checking before going down the diagnostic route which really is the only 100% way of checking.
Agree with Chas on this one, probably a bad wire/connection under drivers or passengers seat. A check on a VAG type computer should give the answer but garages often charge about £50 for this.
Car is safe to drive but the airbag may not trigger in the event of an accident. Also a car will fail the MOT now if a warning light on the dash is lit...
Roll the seats back and forth and see if the light goes out but don't try to fix this yourself unless you are a mechanic, you don't want to trigger the bag by mistake !
Have you checked your battery ? If it's not the battery then as above could be wiring loom under seat. My peugeot had this same problem and after several 'fix it' attempts (including new wiring loom) by local Pug garage it was simply the battery.