Scientific American has an article about fighting spam. Interesing read, intended for less technical audience.
What was not pointed strong enough, in my opinion, is that spam is very cheap to send and that it's very often sent from computers infected by viruses, worms etc (this wasn't even mentioned; because the authors are from Microsoft or it's just a coincidence?).
What is also worth saying is that the most dangerous spam, that's not filtred by many (all?) currently used programs, looks like legitimate messages. For example, the message I got not so long ago looks like a message from ebay. One thing is different - link, that points to fake website. If smap looks like an email from a company or organization that really notifies its customers by email and the person that gets the spam is really its client, the probability for the attack to work is much bigger. Spam may be also sent at the same time when DNS cache poisoning attack is launched (what means, in short, that user entering the right address will be redirected to a wrong server). Then the situation may be really dangerous. Especially, as usual, for the users that are not aware of the danger. So, spam fight will probably take much time...