OK, I've had it with trackbacks.
It's a good idea. You have a blog, I have a blog. I can post a comment your blog by posting on my blog and sending you a trackback. This is great because it means that I own the comment and can update/correct it in accordance with my own ethical standards.
The only trouble is, it's not authenticated, so it opens the door to spammers. I'm seeing that about 99.99% of trackbacks I receive are spam. And 99% of spam that I get arrives via trackback. This is in line with what others are seeing.
And there's not much more that I can see to do about it. They're already being filtered with Akismet. I could add some content scanning or other heuristics but these seem fairly limited at best. I certainly can't add CAPTCHA or similar protien-folding tests.
So, screw it. Bye-bye trackbacks. Come back when you've got some protection against spam.