The usual virus scenario is one of a lone malicious hacker driven mainly by vanity. Virus are usually software graffitti tags. On slashdot however they are speculating about some recent distributed Denial of Service attacks on spam blocking services and their connection to recent heavy viruses. The way this particular conspiracy theory goes, the DDoS attacks on the spam blocking services are being piggybacked on the current SoBig infection.
To the story's discredit it is being propagated by The Register, the most paranoid 'alternative' news source around. But the thought is certainly obvious. Even if SoBig got its start in the usual fashion, it seems like a pretty straighforward thing to create a strain that attacks a particular site.

UPDATED
I don't think the Register is really the origniator of this story. The story is getting coverage from more and more sources, and seems creditable. But please note that some analysts do consider this paranoia. It is simply unnecessary to add the prosecutable crime of spreading a virus to the spamming itself. There are plenty of effective ways to spam.

I wonder what the proper response is. 'More hardware and bandwidth' is probably a losing strategy. If it was possible to distribute the RBL itself somehow, then that might work....