I'm logging hundreds of these events on SBS2003 with SQL Server 2000 SP4. Sometimes five or six events per second.
Anyone got any more information?
I'm currently importing thousands of emails via OMI from old PSTs and the Mail Store.
EDIT: After a little bit of research, this is reasonably normal behaviour for SQL Server Full Text Indexing. You need the associated iFilter for the document type being stored. Adobe have a pdf iFilter for example to enable indexing inside PDF documents.