7. April 2006
I came across this error today:
Database diagram support objects cannot be installed because this database does not have a valid owner. To continue, first use the Files page of the Database Properties dialog box or the ALTER AUTHORIZATION statement to set the database owner to a valid login, then add the database diagram support objects.
Nice and simple to fix this one, change the database compatibility level to 2005.
EDIT: see also http://msdn2.microsoft.com/en-US/library/ms186345.aspx