Euan talks some more SQL Server 2005's XML internals, Michael Rys on Document Instances
Euan Garden that is. More Here.
One of the more interesting Q&As...
What parser is used when manipulating XML inside SQL Server Engine?
XMLRW, which is the new small stream based parser for native code.
Michael Rys has also recently clarified that there will not be an option to have untyped XML instances constrained to just being well formed documents declaratively [e.g. declare @x xml(document)] for SQL Server 2005 but this may appear in future versions. While I'm okay with that, if you have feedback on it, please feel free to post that here and I'll make sure it the team gets it. Of course, you can do it with a check constraint on a column as Darshan posted. Maybe we can talk him into putting that example up on ttp://www.yukonxml.com. Nothing like peer presure, right?