I recently posted this on securence and thought that it might be helpful to my readers:
Unfortunately, spam should be on the rise for the distant future. As technology gets cheaper to send spam, people will send more of it. Eventually, the returns to sending spam will decrease as filters get better, but that'll only create incentives for spammers to send out more emails, because whatever few pieces of email get delivered will be very valuable. You will continue to see a trend towards off-shore emails that are hard to track, and therefore hard to immediately isolate as problematic. On-shore email blasts will continue to become more legitimate looking, and some of these companies will try to break into the opt-in email business. Unfortunately, most spammers won't handle opt-in email with care, and those lists will slowly be handled in the same way that email lists that are not opt-in are handled.
Get more information about spam blocking and the spam business here at the anti spam blog.