I've been implementing this slowly over the last few months. Takes annoyingly long (it's boring), but it's getting there. In the mean time IMAP and POP3 code in 1.0-tests keep stabilizing nicely. So after this one large change things may be a bit unstable for a while, but after that 1.0 should be very near :)
I personally think one small issue in the current dovecot is kind of a showstopper. If a user exceeded their quota, dovecot will just plain not work at all. So a user can also not delete their email to remedy the situation.
This is what happens when a customer (without previously having used dovecot) connects:
A002 SELECT "INBOX" A002 NO Internal error occured. Refer to server log for more information. [2005- 05-31 17:56:36]
The internal error is a 'quota exceeded'.
I think at the very minimum someone should be able to delete email...
Regards,
Cor