actually if you take a look at graph at http://www.symantec.com/business/security_response/landing/spam/index.jspit looks like spam/total messages ratio is constant. only the amount of total messages has dropped.maybe people are abandoning email in general?
...People who will presumably be using Norton's antispam tools, have protection and be aware of fishy emails.
I admit it. I've been on vacation since last august and I've hired trained monkeys to do the job of sending SPAM. Not very efficient these monkeys.