I finally got sick of constantly moderating and deleting trackback and pingback spam (for a definition, see sping at Wikipedia). So I’ve disabled all trackbacks on all posts, old and new. Comments are still gladly accepted and pretty much open. (You should get approved automatically if you have had a comment previously approved; if you’re a first-time commenter, you might have to fill in a Captcha.)
That’s a shame for the legitimate sites that want to use trackbacks for their legitimate purpose, but I’m sorry to say those sites are being outnumbered by about 20:1.
Ed Bott 
We did the same thing a few months ago, Ed. A shame that we don’t have trackbacks to legitimate sources, but they were far outnumbered by the spam. Too bad
I did the same thing about 1.5 years ago… just way too much work involved.
Same here – turned Trackback off a couple of years ago because I was getting about 1 good trackback per month and 100s of spam trackbacks every day. It’s a nice idea but nothing has been done to reduce the spam problem – I was part of a working group to improve the protocol but no-one could agree about how to go forward and the project died.
Ditto here. We turned off trackbacks at chron.com in 2006. It was a sad moment; I love em when they work. But with over 200 blogs, the trackback spam was bogging us down, bigtime. At one point, we were getting 64,000 trackback pings a minute from spambots. Ugh.