Victory!
Well, I think I finally defeated the spam bandits. At least for a while.
The basic problem stems from the complexity of Movable Type's anti-spam technology. The system does an excellent job of identifying and junking the trackback spam. Each one is analyzed in 4 or 5 different ways before it is published. When the system receives a huge volume of trackbacks at the same time, however, it can slow your server to a crawl.
Even when I turned off the "Allow Trackbacks" switch on my posts, the spammers where still able to get their trackbacks into my junk filter. This means that my server was still cranking away doing analysis.
My final solution was to edit the .htaccess file with RedirectMatch and some regular expressions to identify my old posts and point them to a plain old .html document. This bypasses my Movable type scripts all together, and should be much easier on dreamhost's server.
We will see! If you would like details of my hack, let me know. I don't really want to post it here for the enemy to see.