This past week saw a new botnet attack, increasing spam traffic by 600 percent. The botnet responsible is believed to have over 5 million nodes and interesting enough, only works from 9-5. It appears to originate in Poland. It's attack is so over the top experts are considering it akin to Denial of Service Attack.
"The current spamming tactic is to gently deliver spam in a manner that closely resembles legitimate e-mail. From
CRN.com:

"The current spamming tactic is to gently deliver spam in a manner that closely resembles legitimate e-mail. What we saw yesterday was a return to an earlier period, when it was relatively common for spammers to use directory harvesting to build lists of valid e-mail addresses. A new twist on this old technique was to use a bot network to perform the task," said AppRiver executive vice president Scott Cutler in an e-mail received on Tuesday.

Fortunately for email administrators, whoever is behind the botnet left the IP addresses of the nodes exposed, making them easy to block.