Let me start off by saying the initial cause was *not* a Dovecot bug. :)
If you have the default namespace "INBOX." (Courier emulation mode from the Wiki) and an actual folder in your ~/Maildir/ with INBOX in the name (~/Maildir/.INBOX.Miami), then Dovecot presents them both to the client as the real Inbox in Thunderbird:
Account Inbox (3) Drafts Sent INBOX (3)
...where (3) is me representing a new email account; if you click on either INBOX folder (parent or child) they show the same contents of the single Inbox. It was really confusing until I figured out the Maildir was messed up on the backend.
Sometime back in 2004 the Courier-IMAP server let one of my (dovecot) testers create a folder called .INBOX.Miami; until the 1.0rc25 upgrade we never even knew it was there - it has been invisible this whole time. I can only assume that a bug in Courier allowed that folder to be created in the first place, I'm not bothering to track it down.
Once I upgraded to 1.0rc25 this morning all the sudden there it was -- obviously not Dovecot's fault for putting it there and it was easy enough to fix by hand, but I thought I'd report it in case it exposed some logic gap in the code. If nothing else Dovecot shouldn't have duplicated the INBOX view to the client, and instead just showed it as a regular folder - yes?
-te
-- Troy Engel | Systems Engineer Fluid, Inc | http://www.fluid.com