Fixed in IMP 6.2.0beta2 ( pear installation )

http://bugs.horde.org/ticket/13218#c6

Looks like Bug number got lost in commit message... this is the commit 
I was referring to:

https://github.com/horde/horde/commit/f8e275b289d8314fcfa5ad66d0a8609f02c2a31e

Pretty sure this was fixed.  Specifically, this change (we were 
stripping namespace prefix from all mailboxes, not just personal 
namespaces):

--- a/imp/lib/Ftree/Account/Imap.php
+++ b/imp/lib/Ftree/Account/Imap.php
@@ -112,23 +112,23 @@ class IMP_Ftree_Account_Imap extends IMP_Ftree_Account
               * step through the name to make sure all subfolders exist
               * in the tree. */
              if (strlen($val['delimiter'])) {
-                /* Strip personal namespace. */
-                if (!empty($ns_info['name']) &&
-                    (strpos($mbox, $ns_info['name']) === 0)) {
-                    $parts = explode($val['delimiter'], substr($mbox, 
strlen($n
s_info['name'])));
-                    $parts[0] = $ns_info['name'] . $parts[0];
+                /* Strip personal namespace (if non-empty). */
+                if (($ns_info->type === $ns_info::NS_PERSONAL) &&
+                    strlen($ns_info->name)) {
+                    $parts = explode($val['delimiter'], 
$ns_info->stripNamespac
e($mbox));
+                    $parts[0] = $ns_info->name . $parts[0];

Obviously, this patch can't be applied directly since it depends on a 
bunch of other changes (i.e. in Horde_Imap_Client) but I now see the 
namespace prefix appearing in my Shared mailboxes folder.

Turning 'tree_view' value to '1' in IMP prefs : shared mailboxes are 
now displayed in a separate folder called "Other users" but not the 
real name of the namespace from the IMAP server ( userprefix parameter 
in imapd.conf ).

When listing mailboxes, shared mailboxes ( mailboxes from Other Users 
namespace ) are on the same level of INBOX : they are not shown inside 
a "Other Users" container.