Someone forwarded me a blog post with a pretty thorough recounting of their experiences upgrading SQL in a TFS install.  This is the kind of thing we have worked very hard to make better in 2010.  While the SQL and Sharepoint parts of such config changes still have their challenges (and are getting better in their own right), we've made the TFS part WAY better.  Anyway, it's a pretty good read if you need to do this too.

http://nullabletype.com/2010/02/25/migrating-the-tfs-2008-data-tier-from-sql-server-2005-to-sql-server-2008/

Brian