There should be an article about using SpamAssassin at the gateway level - i.e. on a Sendmail box running as a front-end gateway for a number of other mail-servers, and filtering mail at this level, before forwarding.

I've looked all over the netfor a good explanation/walkthrough on this, but have yet to find one, and have actually had to decide to use a third-party spam-filtering service instead (MessageLabs.com - which incidentally is very good, and not too expensive).