After that disapointement, I kept my 8 years old mail setup, being fetchmail running on my workstation and throwing the mails in procmail, then using mutt locally to read the maildirs. But that's over.
I got a laptop to replace my workstation. It was not possible to continue using such a mail setup, since my laptop can be offline, and so would be my mails.
So I decided to upgrade Dovecot to 1.2. I used the dovecot-1.2-work Subversion branch of our lovely Debian maintainers, and built a Debian package for Lenny. The upgrade from 1.1 was almost painless, since the configuration file did not change heavily.
Then I started to write my little Sieve script. Sieve is a very nice language. Almost user friendly. So in 20 lines I rewrote all my procmail stuff, matching things like List-Id with regex to put the mails automagically in the right folder. I reconfigured mutt to use IMAP, and it works fine. I even reimported my old Maildir via IMAP using mutt too.
I am now a happy IMAP user.
For people wondering why I wanted to switch away from procmail to Sieve: the reason is that Sieve script can be uploaded remotely via managesieve. This means you do not need FTP/SSH/whatever access to put your script. You can, for example, use connect-sieve or the Sieve plugin for Thunderbird/Icedove.