We ship a "Microsoft Best Practices: Security" policy called "Guest Permissions on User Databases".  Unfortunately, the wording in the UI and BOL is not clear about whether this can/should apply to msdb.  

Here is what is says today:

"You can do this by executing REVOKE CONNECT FROM GUEST" from within any database other than master or tempdb."

Here is what it should be (and will be in the next major release):

"You can do this by executing REVOKE CONNECT FROM GUEST" from within any database other than master, msdb, or tempdb."

For more information on the "guest account in msdb" issue, see Buck Woody's recent post titled "Don’t mess with the system databases in SQL Server, or Error: 916"

Regards,
Clifford Dibble
SQL PM