User mail spool partitioning mystery

[Patrick Goetz]

I have a couple of existing cyrus 2.3.16 installs with this partitioning 
configuration in imapd.conf:

   defaultpartition: default
   partition-default: /home/cyrus/mail

I create users using cyradm:  cm user.myuser

and the user mail spool folder has this heirarchy:

   mail
     |
       a
       |---- user
               |---- auser1
               |---- auser2
               |---- auser3
              ...
       b
       |---- user
               |---- buser1
               |---- buser2
              ...
       c
       d
      ...
       x
       y
       z
       stage.


I just assumed that this structure was built in to cyrus; however on my 
new 2.4.17 install the partition settings in imapd.conf are similar:

   defaultpartition: default
   partition-default: /srv/cyrus
   unixhierarchysep: yes   <--- (now using this)


however all newly created users (cyradm:  cm user/myuser) are dumped 
into a single folder:

   cyrus
     |
       user
         |---- myuser
         |---- otheruser
         |---- thirduser
        ...


I simply can't find any difference in the configuration files that 
result in this discrepancy.  Was the [a-z] partitioning in the 2.3.16 
install baked in to the Debian cyrus package I used, say in cyradm?