Regarding Plesk/CPanel you might consider looking at the open-source project we released:

http://symbiosis.bytemark.co.uk/

As you can see it is thoroughly documented and allows you to create websites and mailboxes via SFTP. For example to create the hosting for the site "example.com", and some mailboxes you'd run:

   mkdir -p /srv/example.com/public/htdocs
   mkdir -p /srv/example.com/config
   echo "ftp-pass" > /srv/example.com/config/ftp-password

   mkdir /srv/example.com/mailboxes/bob
   echo "secret" > example.com/mailboxes/bob/password

Built around Debian Squeeze, iwth wheezy support in the future, it is a modular series of packages for IMAP, POP3, Webmail, Website hosting, with utilities for SSL, etc, too.

Adding a control panel would be trivial, but we tend to assume folk will login via ssh/sftp.