Matt Cutts wrote a short post on how Google is doing it's part in reducing Blog Spam, citing Destroy all Malware.

The point I get out of the Malware post - the ability to react quickly is what is taking the wind out of blog spam while email spam is still growing because the entities that control email are different than those who control search engines - read the Malware post to see what I mean.

"If you compare the growth of email spam to the growth of blog spam, then you'll see two completely different curves. Email spam, even after years of growth, is still growing, whereas blog spam has already peaked. The difference is that email hubs are mostly controlled by large corporations and government, which are not quick to react and spammers easily out-maneuver them. On the other hand, the blogosphere hubs are controlled by small and nimble companies (Google was a start-up on 5 years ago, Technorati, Wordpress), which are capable of keeping up and even out-maneuvering spammers."

Also noted, the sharp increase in comment spam.  I get so much comment spam on all my blogs - 99% of the time the filters in Movable Type and WordPress to their job - but the hackers and spammers are able to outsmart them - so one always has to keep a look out to make sure blogs are not overrun with comment spam.