This happened to my domains as well, and it started right around the same time the original poster reported it.
IP addresses are varied, some are in central america, some in china.
Subject lines started out as "girlie messages", then progressed into job offers.
I don't have the original subject lines from late May 2011, but some recent ones are:
New ladie profiles
Job ad - see details! Sent through Search engine
Job opportunity - hurry to apply!
This is a massive spam effort -- I've never seen anything like it in my ~10 years with Dreamhost. DH's spam filters are not catching these, even though they seem extremely obvious.
BTW -- I should point out that these messages are spoofing the "from" address as if it is originating from my DH-hosted domain. So if the spam is sent to "email@example.com", it's also coming from "firstname.lastname@example.org".
I'm not sure why the DH spam filter can't catch such obvious forgeries.