Kris, as Dag says, trackbacks are the best way to get a lot of spam on your blog. I disabled the trackback module last year because I got flooded with spam. To test the current situation, I enabled trackback again Thursday evening. Result: 310 trackback spams in the moderation queue in less than two days!
I prefer to install a decent antispam and enable anonymous comments without moderation in that case. Since my antispam is active (Oct 2007), 18527 spams got blocked or more than 150 a day (yes, spammers seem to love me for some strange reason. *sigh*). If it would be able to filter trackbacks, I would reconsider to enable trackback, but right now it's absolutely out of the question.
Not sure if Drupal does this, but it might be a good idea to allow trackbacks only from certain trusted blogs. If Drupal doesn't do this, then it can't be all that hard to implement.
i'm using wordpress + akismet on my blog, and don't have trackback-spam issues, akismet filters them all out nicely. i'm sure akismet can be integrated in drupal (for comments and trackbacks) as well?
There is an Akismet module for Drupal: http://drupal.org/project/akismet . I don't have any experience with it, but I took a quick look at the settings and I don't think it supports trackback filtering. (I heard that the Drupal Trackback module doesn't have an API to interact with antispam modules.)
that's ... weird, no? i mean, aren't trackbacks just a specific form of comment? i don't really know drupal, but shouldn't the trackback module be an extension of the comment module, enabling it to use e.g. akismet as well?
Trackback is a contributed module, so the exact implementation is chosen by the author. The trackbacks go into separate database tables and can't be seen as comments here...