Here's a thread about it on vBulletin.com.
http://www.vbulletin.com/forum/showthread.php?t=130527

Basically, the spammers are using BOTS to sign-up to vBulletin messageboards automatically. Even with image-verification turned on, they are somehow getting through. Eventually they post porn spam to your forum. I've been seeing this at one of the forums I run, and its very irritating. I've seen conversations beginning to pop up around the net, related to this rising new issue. Right now, I have the impression these spammers are "seeding" as many boards as they can before pulling the "trigger" and going on a posting frenzy.

I've blocked the following IP addresses:
200.73.174.183
12.216.140.10
12.202.237.194
66.218.19.68
12.207.135.72

I've also blocked the following e-mail domain names:
hottamlideals.biz
lacedwhitepanties.biz
lightblondevagina.biz
bakedpotato2005.biz
bagelsandcreamcheese.biz

I was a bit stung to read one of the vBulletin developers note the following as a solution:
I would add this to the Email Ban list:
.biz

~ Steve Machol
vBulletin Team
Support Manager
A lot of scarey "baby out with the bath water" solutions going around. The level of paranoid intolerance will probably worsen as spam continues to rise to new levels. The Net @ large has gotta get a grip, or things will start splitting at the seams.

~ Nexus