Aargh, I've have spent entirely too much time cleaning up after some faceless and nameless admins. For whatever reason, there was a decision to install courierimap, which by default creates 'maildir' style mailbox folders. The previous imap server ran fine for me. I actually liked the 'mbox' style folders. It was very easy to locate the explicit path to your inbox. Once you understand the location of the inbox, writing procmail recipies are a breeze.
Now that the mbox format has been replaced in favor of the allegedly improved 'maildir' (there is supposed to be some speed improvement for very large files), which has the cur/, new/, and tmp/ subdirectories. All of the mail is spread all over the place. What a mess.
It seems that the average user on the virtual domain which I lease do not use procmail, so the admins don't really seem to care that they screw everything up each time there is an attempt to upgrade imap servers.
After spending a couple hrs reading, I modified my .procmailrc and added '.' and '/' to each of my folders. It seems that maildir style boxes adds a '.' to the beginning and a '/' to the end of your folder. So your procmail script must also contain these attributes. The $DEFAULT variable must also look something like this - $DEFAULT=$HOME/Maildir/ The trailing '/' is significant, as it explicity tells procmail that you are using 'maildir' style mailboxes and not the ubiquitous 'mbox' format.
Yes, I sort everything which touches my inbox, so it really isn't cool for procmail not to work as desired. More on this debacle later.