I have started seeing spam messages getting though my filter. It looks like it is being cause because the spammers a sending mail from computers that have not been listed as untrusted relays. So as spammer are taking over more and more zombie PCs, the ALL_TRUSTED rule is being triggered.
The ALL_TRUSTED rule decreases the spam score when it determines that the mail did not go through any "untrusted" computers.
I have found this page that suggests adjusting the score values for this rule. I adjusted the score for this rule by 2. So now the scores for ALL_TRUSTED look like:
score ALL_TRUSTED -0.400 -0.820 -0.867 -1.300