It is likely caused by a botnet infecting many people, but I haven't seen any reports of it on any news sites.
The total volume of email has increased a lot in the last 2 months, and with that a steady increase in spam slipping through the filters. The amazing thing is that some look like the same email over and over, which you would think the filters would catch. I suspect that each one is slightly different in just enough of a way to get past the filters.
This is not just affecting our servers. I have noticed a lot getting through Google's Postini service, which is a major spam blocking service. So, if Postini is have troubles keeping up, then I can't bang my head too hard.
I'll be tweaking the filters and server settings to see if I can tighten up a little, but the usual process is that the filters and blacklists catch up and start blocking better. Right up to when the spammers find another way to attack.