I've just updated from CVS and it is working fine in IMP/DIMP.
Thanks.

I can't tell you what program did create that folder. I just know that 
a few users here have it.
I don't know if the folder name is valid, but I know that the IMAP 
server and other apps like thunderbir/outlook do handle it. Here is 
the output of the IMAP server :
a003 list "Folders" "*"
a003 OK LIST completed
a003 list "Folders" "*"
a003 OK LIST completed
a005 list "Inbox" "*"
...
* LIST (\HasNoChildren) "." "INBOX.INBOX.Ronan"
* LIST (\HasNoChildren) "." "INBOX.INBOX.Sent"
* LIST (\HasNoChildren) "." "INBOX.Sent"
* LIST (\HasNoChildren) "." "INBOX.Notes"
...

I reported the bug because I thought that there was one, and event 
this type of naming doesn't usually happen, it can happen.

How is that a legal mailbox name?  In that mailbox, your base 
namespace would necessarily have to be '' (blank).  As such, that 
mailbox is correctly identified as 'INBOX.Sent', NOT '.INBOX.Sent'.   
You simply *can't* have an empty mailbox name.  See RFC 3501 [7.2.2]:

     The hierarchy delimiter is a character used to delimit levels of 
hierarchy in a mailbox name.

By definition, a mailbox name is *1* or more characters.  See RFC 3501 [9]:
mailbox         = "INBOX" / astring
astring         = 1*ASTRING-CHAR / string
ASTRING-CHAR   = ATOM-CHAR / resp-specials

Additionally, this mailbox requires that its parent is the mailbox ''. 
  However, that mailbox name is reserved by list as a method to 
determine what the hierarchy delimiter is for a given reference (RFC 
3501 [6.3.8]):

       An empty ("" string) mailbox name argument is a special request to
       return the hierarchy delimiter and the root name of the name given
       in the reference.

Your mailbox is invalid.  I don't see a need to workaround invalid 
mailbox names.  I don't care if other mail programs can handle it or 
not either.

following Bug: 6951

I have a folder called ".INBOX.Sent". It appears like this in IMP :
- INBOX
       +-- Sent
- INBOX
...

the second INBOX is not clickable.
The expected tree is :
- INBOX
+-- INBOX
       +-- Sent