DM Review’s 2003 article on Automatic Classification is a good intro to the various types of auto-classification: rules-based, supervised learning and unsupervised learning.
CMS Review has a good list of Metadata Tagging Tools and a list of other resources at the end.
Taxonomy Strategies provide a bibliography on info-retrieval that includes automatic classification articles.
From 2004 there’s the AMeGA project
Delphi’s white paper ‘Information Intelligence: Intelligent Classification and the Enterprise Taxonomy Practice’. Download from Delphi’s whitepaper request form.