Referring again back to the article about 29% of internet users buying stuff from there spam, here's another excerpt which is less controversial:

Research and Content Engineering) security team, indicates that global spam volumes doubled for the year ending June 2008.

The company attributed the rise largely to botnets, or networks comprised of thousands of infected personal computers, controlled remotely by criminals.

“They have enabled spammers to push down their costs through economies of scale and eliminated the need for spammers to host their own spam servers as they simply take control of other people’s computers instead,” the company said in a published statement. “Recent FBI prosecutions of bot-herders and investigations of message-boards used by spammers, suggests the going rate for spammers to send a million spam messages is as little as $5 - $10.”

According to Marshal, its research indicates that just five botnets account for 80% of the world’s spam.

I'm going to comment on the two underlined points, both of which are emphasized by me.

1. Our own stats indicate that inbound spam volume has not doubled this year, in fact, we have seen the opposite - spam volumes have significantly dropped throughout the year with slight week-over-week upticks, but the trend remains intact.
   
   I don't know why spam is dropping; maybe some of the biggest spammers are going to jail, maybe Microsoft's Malicious Software Removal Tool is effective at getting some PC's out of botnets, maybe some spammers are giving up, or maybe our stats are an anomaly... the point is that we aren't seeing this trend.  What we do see, however, is short bursts of effective spam campaigns like the CNN spam of two weeks back.
   
   
2. The second point is interesting, most spam is sent from 5 botnets.  It looks like the big bot herders are starting to consolidate their interests and push out the smaller botnet operators.  Bot herding is good business so it makes sense that the big players would move to get rid of the less organized bot'ters.  I guess there is no room for the mom and pop bot herders anymore.