Heidi Adkisson has posted on the Use of Faceted Classification in Web Design Practices which examines how e-commerce sites use faceted classification to help people find products.
A series of papers are available in Kathryn La Barre’s project website: Facets on the Web from the School of Library and Information Science, Indiana University.
A definition of Faceted Classification can be found in the IAWiki page: FacetedClassification
Update: (05 Jul 2006)
Another useful page I’ve come across on faceted classification.