NYT [via news.com] [click through ad to read article] has a roundup article on the state of spam ("Spam Doubles, Finding New Ways to Deliver Itself") [NYT link here].  The article notes the three main spam-filtering strategies and how "Spam 2.0" is frustrating all three of those strategies.

Antispam companies fought the scourge successfully, for a time, with a blend of three filtering strategies. Their software scanned each e-mail and looked at from whom the message was coming, what words it contained and to which Web sites it linked. The new breed of spam--call it Spam 2.0--poses a serious challenge to each of those three approaches. 

First, the article notes the increased use of bots and viruses, which are used to send messages from innocent computers.  (See, e.g., ars technica here ("Botnets Cause Significant Surge in Spam").)  This makes it difficult to block based on the senders' IDs.  Second, the spammers switched to image spam – where the emails contain no text, but purely graphics.  Finally, the link-based filter (or enforcement) strategy was frustrated by the fact that the latest breed of spammers are engaged in pump and dump stock scams.  They are not selling anything at all.  The article also notes that filters were trained to fingerprint spam and filtered based on common fingerprints.  The spammers started changing something slight in every message (even one character) so the messages were not exactly the same. 

The article then wonders what the future holds in store.  According to those polled, it does not look bright: 

The bad guys are simply outrunning most of the technology out there today.

The last point is an interesting one.  If there's one situation in which one would think the private sector would step in and solve the problem this would be it. 

We'll see what the future holds in store.