Backported from CVS HEAD to branch_1_0. You can get it as a patch from http://dovecot.org/patches/1.0/quota-rewrite.diff
New features include support for multiple quota roots, so you could have eg. per-user quota and a larger domain quota (although no backend actually supports this yet..).
The more important feature is that you can configure specific mailboxes to have a bit more space available for them, such as Trash or Spam. The configuration has also changed. Normally you'd use something like this:
quota = maildir # 1GB quota in general quota_rule = *:storage=1048576 # Trash mailbox gets 100MB more (so max. 1,1GB) quota_rule2 = Trash:storage=102400 # Spam mailbox gets also +100MB (still maxes at 1,1GB, not 1,2GB) quota_rule3 = Spam:storage=102400
You can place all the above into plugin { .. } section in dovecot.conf. In userdb if users have different quotas you'd then need to override only quota_rule (not quota, quota_rule2 and quota_rule3).
I tested quickly that this seemed to work as described above. Please test and report. :)