[Dovecot] How do I use plus-addressing in Dovecot 1.1.x?

[Jon Fullmer]

I'm using Postfix, Amavisd-new/SpamAssassin, and Dovecot. They all run  
on the same server (running Linux). The mailboxes are stored on the  
same server in maildir format.

The logical path for incoming messages (as I understand it) is:

1) received by Postfix; handed off to Amavisd-new
2) Amavisd-new checks to see whether or not it's spam/undesirable;  
hands off to Dovecot deliver
3) Dovecot deliver delivers the message to the local mailbox

Under each of my user's mailbox is a submailbox called "spam". Just so  
I don't confuse terminologies (I'm no IMAP/maildir expert), for  
example, we have someone at domain.com. My configured mail_location is  
"maildir:/srv/mail/%d/%n", so this person's mailbox is located here:  / 
srv/mail/domain.com/someone/   Within this directory is another  
mailbox:  /srv/mail/domain.com/someone/.spam/

The plan is to have Dovecot deliver send a message (which has been  
labeled by Amavisd-new as spam) directly to the ".spam" submailbox,  
instead of the INBOX (/srv/mail/domain.com/someone/). With Dovecot  
1.0.x, I used "plus addressing". Amavisd-new was configured so that,  
if a message was found to be spam, it delivered the message to Dovecot  
deliver, but changed the delivery address. Now, instead of "someone at domain.com 
", it was "someone+spam at domain.com". Dovecot deliver took this message  
and delivered it correctly to the ".spam" submailbox (/srv/mail/ 
domain.com/someone/.spam/). Just to emphasize, this process does work  
(using Dovecot 1.0.x).

With Dovecot 1.1.0, this process is broken. Now, spam messages are  
delivered to the INBOX instead, and, when checking the dovecot- 
deliver.log, I'm seeing "save failed to spam: Unknown namespace".

As best as I can tell, the answer seems to lie in "namespaces" as a  
feature added in Dovecot 1.1.0. The problem is that I don't understand  
how I could use them to resolve this situation. I've read the  
documentation. I've looked at as many example configurations as I  
could find.

Please help. I want to get this working again. I'm sure it's something  
very straight-forward, but I don't understand. How must I change my  
Dovecot 1.1.x configuration so that my "plus addressing" set up  
(detailed above) will work again?

Or is there a better way?

Thanks so much for your help, and thanks for a great product!

  - Jon