I ran into a wierd error message when trying to view the Diagrams on a database that was migrated to SQL Server 2005. Something about the user not being valid and to change the database owner.
A quick search brought up this quick fix:
Try running this command to change the compatibility level to SQL Server 2005.
EXEC sp_dbcmptlevel database_name, 90
Works like a charm! [SQL BOL]