I'm sure you've already seen these items, but I thought I would pass along just in case they are useful or anyone else needs them:
http://blogs.msdn.com/brianhartman/archive/2008/11/21/custom-credentials-in-the-report-viewer.aspx
http://blogs.msdn.com/jameswu/archive/2008/07/15/anonymous-access-in-sql-rs-2008.aspx
http://technet.microsoft.com/en-us/library/ms159272%28SQL.90%29.aspx
http://blogs.msdn.com/microsoftbob/archive/2009/08/20/a-wrapper-for-running-sql-server-2008-reporting-services-reports-anonymously.aspx