Here's the BBC report: 'The vast majority of spam is sent by networks of infected computers known as botnets.'
I love this comment from Symantec:
The main cause of this drop is a from a huge reduction in output from the Rustock botnet, by far the most dominant spam botnet of 2010. Since 25th December, Rustock seems to have all but shut down, with the amount of spam coming from it consistently accounting for below 0.5% of all spam worldwide. Further contributing to the massive reduction in spam levels is the apparent mollification of two other major botnets, Lethic and Xarvester. MessageLabs Intelligence has seen virtually nothing from Lethic since the 28th December, and Xarvester since the 31st December.
As someone on metafilter says, this reads like a quote from A Fire Upon the Deep.