I purchased a 2001 996 c2 and the seat belts were definitely in need of cleaning as well. I posted the question on PCA's web site and received this answer.
"The fabric is synthetic and is prone to staining. Products such as Afta will remove grease from fabrics with no danger of harming the belt. Afta is available at hardware stores.
John Paterek - PCA WebSite - 9/11/2005"
BTW my 996 is Speed Yellow and seat belts are as well
