I run SpamAssassin in its Daemon mode and then use a procmail pre-filter for some of the lists (the ones that are open for anyone to post).
Instead of the list names going directly to the mailman aliases (calling the wrapper program), they call a procmail script which does some checks (including a trip through spamd) and modifications to the message header. If the message passes through my checks then the script passes the modified message on to mailman's wrapper.
This keeps viruses out of the mailing lists and minimizes the spam. It also adds about one minute to list delivery times, and takes away a chunk or RAM: ~64Mb base plus 9Mb for each concurrent message being checked.
Note: please send these requests on-list. I run a consulting business and I have to charge for individual attention (or I and my family will eventually starve). Plus I normally answer on list, and you get the added benefit of a world-wide network chipping in with better solutions!
Hope this has been helpful - Jon Carnes