This article was prompted by a question from the audience about the internals of XML Indexes. In particular, they were interested in where the internal system table used by Primary XML Indexes is stored, and what might happen to this index if the database is detached and then reattached on another server.
http://www.jasonfollas.com/wiki/SQLXML_Tricks_XMLIndex.ashx
Note: I will likely continue to expand on this particular article to include information about secondary indexes and when to use each type.