I’ve been noticing a lot more spam getting through my server-side filters and also passing through my client-side filters lately. It’s apparently not my imagination:
A significant rise in the global volume of spam in the past two months has security analysts worried that bot nets are increasingly being used by spammers to stymie network defenses erected to curtail bulk e-mail.
[…]
While bulk e-mailers have, in the past, sent unwanted messages from a single server, increasingly the spam emanates from networks of compromised PCs, known as bot nets. The level of junk e-mail has increased almost in lock step with the number of compromised systems used for spam, said David Hart, the administrator for Total Quality Management.
“What is most alarming is that new clients–Internet addresses that we have never seen before and which could be new infections–have tripled since June,” said Hart, who posted a chart tracking the growth on his Web site this week.
The chart is fascinating:
It looks like the surge started on June 11.
Hmmm. Couldn’t have anything to do with this, could it?
Ed Bott (@ wordpress.com) 
After reading your post I looked at our spam stats. We average 80% spam and about 10-15K messages a day. On 10/21 we spiked to 25K and then on 10/24 it went to 40K. I am not sure what is happening but it is growing fast. I would wonder if some servers won’t be able to handle this load increase?
I too looked at my spam stats, and my big increase was the week of March 26. Spam doubled over the week of the 19th. We’re now getting about five times the spam per week we were getting in March. My guess is that Office vulnerability MS06-12, announced on 3/14/06, is the culprit. We’re up to about 65% spam from as low as 25% earlier in the year.