I have the same problem on a 99 1.8LX
Came on for about 10000 miles then went off for a while. Now it's on all the time again. Asked in the garage, they ran diagnostics and said all was fine with the airbag and that the problem was most likely a bad sensor which they replaced ... to no avail. I solved the problem last week by putting a bit of black insulating tape over the light. I know its still on but at least i can't see it. My car has passed MOT twice with the light on so I can live with it.
John M, August 2009