Google's chief spamfighter says that he's seeing less spam email coming into Google Mail and speculates that spammers are giving up on bulk spam because of the efficacy of filters; he predicts an enormous rise in quasi-spam from companies that you did business with once or twice and now feel entitled to email you all the time. I get craploads of this stuff myself — PR releases (any PR person who puts me on a mailing list goes straight into my killfile, forever), circulars from some etailer I bought something from eight years ago, even monthly newsletters from a clinical massage place in Toronto I saw for a sore shoulder, once, in 1988. Unsubcribing to this stuff is time-consuming and only works about a third of the time, in my experience.
Of course, I also get thousands of spams a day (most of which are successfully repelled by greylisting, leaving only a thousand or so that get through to my mailer, which filters all but a couple hundred).
Google won't disclose numbers, but the company says that spam attempts, as a percentage of e-mail that's transmitted through its Gmail system, have waned over the last year. That could indicate that some spammers have gotten discouraged and have stopped trying to get through Google's spam filters.