Here’s an interesting tidbit from MessageLabs’s september report: on the day that the alleged malware hoster Intercage was taken offline, there was a significant drop in malware activity: 

The drop was very shortlived; this suggests that Intercage was primarily hosting C&C (command and control) servers that control the activity of the botnets used to send spam. MessageLabs agrees with this:

It can be seen from the chart above that the botnet controllers are quick to respond to any degradation of their service, and can re-point their bots at a new command and control channel in a matter of days.  Therefore MessageLabs expects this decline in spam to be short-lived, especially in anticipation of Halloween in October and Thanksgiving in the US in November, both of which are traditionally seasonal favorites for spammers.

You can find the complete report here.