...use RBLs at SMTP-Level without any kind of scoring algorithm (only block when $x out of $y RBLs have the IP listed) unless you don't care about your mails. There have been major fuckups with single RBLs in the past and there will be such in the future. Especially with SORBS. See http://www.google.com/search?q=sorbs+sucks.
I thought that'd be common knowledge by now, but apparently I'm mistaken.