I've spent some time writing a document on how to setup your own anti-spam, anti-virus host using most of the components we're all used too - Linux, spamassassin, clamav, etc. I just figured it'd be nice to have it all in one place instead of trawling around the net like I did.
The document can be downloaded here http://www.unimatrixzero.plus.com/OpenSourceBastionHost.pdf
Enjoy.
Andy