This was just a thought, but if each of us changed the file name of our wp-trackback.php file (and changed all references to said file to its new name in the WP-files), would that stop trackback spam? I'm guessing that the only way the spambots know that the URI for the trackback is the trackback URI, is because it has the word "trackback" in it. I am somewhat of a n00b when it comes to programming so I may be completely wrong here.
If this concept would work, perhaps we could see something in a future version of WP that would automate this process (at least partially). Just a thought, here.
If that wouldn't work, could we just hide the trackback URI inside a link tag? Again, I'm new to this side of things and am a little fuzzy on how spambots work. I look forward to any thoughts/flames.