For some reason the munin packages in debian sarge don't seem to include the spamstats plugin.
I fetched the latest spamstats.in file from CVS - this is not ready to run as it needs to be processed by the build scripts - so to install into an existing binary install:
Get the file
Once this is in place
cd /etc/munin/plugins ln -s /usr/share/munin/plugins/spamstats
Then edit /etc/munin/plugin-conf.d/munin-mode - I'm using exim4 - which logs to /var/log/mail.log - and with root.adm ownership - so I used the following:
[spamstats] group adm env.logfile mail.log
env.logdir and env.logfile are used to specifiy which logfile.
Now - restart the munin-node process - and you're done.