I've just finished the final component of the conversion of the backup system - a new restore utility.
There were a couple of problems with the old restore utility, stemming from its design of just copying the messages back into the folder and reconstructing to make them "appear" - it didn't work safely with some IMAP clients, it didn't work safely with replication and there was a possibility that you could delete messages that were already there if you forced the restore to overwrite what was there.
Oh - and a restore could easily push you over quota if you were near the edge as well.
So - the new restore system is much simpler and safer for all cases, though it involves a little more manual work:
Where "username" is the localpart of your login name, and YYYYMMDDTHHMMSS is a timestamp in year, month, day, hour, minute, second format with a 'T' in the middle.
This folder will be deleted automatically one week later. It doesn't count towards your quota usage during that time, though any emails copied out to a normal folder will of course count as usual.
(I've started a forum thread as well for discussion: http://www.emaildiscussions.com/showthread.php?t=51151)