A few days ago, three nonsense-ridden blogs were created in my system. They only published one post each, and it seemed 'automatic' text culled from the web. I see from my logs several entries from the same domain of the e-mails they used to sign up:
http://theirspammydomain.com/wpPoster/aprove.php?url=mywpmuservie.com/ theirusername&step=1&keyword=whatever
So, this /wpPoster seems a script to semi-automatically set up a wpmu account... You may want to check your own logs to see if they're on your system, too!
I've marked them as spam... Do you know if the wpmu system to spot 'splogs' will take care of them from now on?