I just got several instances of what I believe is another resourceful form of blog comment spam. It looked like an ordinar spam, somehow making it through MT-Blacklist system I've got installed and after "Name: free government grants" I was aready clicking on "De-spam using MT-Blacklist" link, but then I realized the domain name to be banned is "journalism.nyu.edu". Hmmm, free government grants on nyu.edu site???? Wait a minute!
And yes, that wasn't a joke. That linked page at journalism.nyu.edu is a very serious political blog rant with lots of comments and obviously "free government grants" comment among them! So here is how I think it works: they post an evil spam comment to a trustworthy blog B. Then if it doesn't get cleaned soon, chances are high that it will be staying in archives for a long time, so they start to spread more evil spam comments linking to the infected page at the blog B.
The bad thing is that banning such spam you have to ban (trustworthy) site B, which can be actually even your friend's site. Ergo: clean your blogs guys, don't keep spam comments in archives.
Things are much more complicated!