We mainly use FB mailboxes internally as a mechanism for linking email and FogBugz; eg send an email explaining a problem or fix and cc FB to add this to the case history.
One of the consequences of this approach is that typos in the email subject, or simply forgetting to add (or correctly format) the "(Case xxxx)" reference, leads to creation of spurious new bugs which then get resolved as dups but we lose some of the context along the way.
It would be nice to be able to designate certain mailboxes as unable to create cases - if you send an email to this address which would create a new bug then you get a templated error response back instead of a new bug being created.