Thomas, for my spam filtering I use Bogofilter in combination with maildrop on my server. I followed this recipe that I improved with my own setup and since I mostly forgot I was receiving spam.

The only caveat is training the filter for false negative, what I just to is move them to a "Spambox" folder and run bogofilter -s -B * in there. And for those who wonder, I didn't feel the CPU usage on my P2 @266MHz. Beside patching courier IMAP to have hooks for the Junk/NonJunk flag and have Evolution actually set them (Thunderbird does it) I don't see another easy solution.