Are you sure it's the seatbelt light and not the airbag light? If seats are changed out without first disconnecting the battery, the airbag light will be set off, at which point the light must be disable by someone with a PST2 (like the dealer) or the Durametric software.
It is definately the seatbelt light. I've had the airbag light problem you describe before and had to go tot he stealer to get it turned off. Thanks for asking