I have noticed that when I enabled read-only mode to the Content database before running the export stsadm command, there will be "access denied" occurring during the export. If the database is not set to read-only mode, then no errors are encountered.
The question now is whether it is safe to perform the export command without setting the database to read-only? Understand that post SP2 backup operation will automatically set it to read only, but not for the export command.