Use at your own risk, but this is how I setup logwatch on Ubuntu 8.04 lts with ZCS 6.0.5

As root:

apt-get install logwatch
update-rc.d -f postfix remove
mv /usr/sbin/sendmail /usr/sbin/sendmailbackup
ln -s /opt/zimbra/postfix/sbin/sendmail /usr/sbin/sendmail
vi /etc/cron.daily/00logwatch

modify the line: /usr/sbin/logwatch --mailto
to look like this:
/usr/sbin/logwatch --detail 2 --output html --hostname yourservername --logdir /opt/zimbra/log --mailto

see man logwatch to play with settings to suit your needs

I found adding the zimbra log directory adds clam-updates to the report