The New York Times has an article on the growth of spam. Interesting facts include: 9/10 of all email is spam, spam source identification is nearly useless due to botnet spam senders, and image based spam (emails which consist of an image only) are on the growth.
Estimates of the cost of spam are almost certainly far to low, because they do not account for the cost in time lost by people.
The image based spam which is currently penetrating many filters should be catchable with a more sophisticated application of machine learning technology. For the spam I see, the rendered images come in only a few formats, which would be easy to recognize via a support vector machine (with RBF kernel), neural network, or even nearest-neighbor architecture. The mechanics of setting this up to run efficiently is the only real challenge. This is the next step in the spam war.
The response to this system is to make the image based spam even more random. We should (essentially) expect to see Captcha spam, and our inability to recognize captcha spam should persist as long as the vision problem is not solved. This hopefully degrades the value of spam to the spammers, but it may not make the value of spam nonzero.
Solutions beyond machine learning may be necessary. One simple economic solution is to transfer from first time sender to receiver a small amount (10 cents?) in a verifiable manner. If the receiver classifies the email as spam then the charge repeats on the next receipt, and otherwise it goes away.
There are several difficulties with this approach: How do you change a huge system in heavy use which no one controls? How do you deal with mailing lists? These problems appear surmountable. For example, we could extend the mail protocol to include a payment system (using the “X-” lines) and use the existence of a payment as a feature in existing spam-or-not prediction systems. Over time, this feature may become the most useful feature encouraging every legitimate email user to offer a small payment with the first email to a recipient.