I've heard anecdotal evidence that spam has decreased. If so, it is likely because of the economy. After all spamming is a business.
I think more precisely, spamming is a double-con... the spam business cons con-artists into thinking they can make money by using spam to con some tiny but significant percentage of the public... a lot of the spam I see is clearly targeted at old people ("Top of the morning to you!") who are new to the net but not wised-up yet. I have my doubts that there are a lot of people like this left.