I haven't really had much of a problem with trackback spam since I installed this trackback validator plugin. However, as their plugin is written, even though it stops the spam, it still e-mails you about it, which is quite annoying (I gave up wading through the wordpress plugin architecture to figure out why it was still e-mailing).
Making things worse was that I was getting hit by trackback spam by a large botnet (over 100 IPs and counting), most of which were listed on cbl.abuseat.org. It wasn't getting through, but I was getting 1-2 email notifications an hour, nonetheless.
So, at Chris's prodding, I went ahead and hacked up this little plugin to check DNSBLs. I am currently checking cbl.abuseat.org and list.dsbl.org for now. If any of you experience any problems leaving comments, let me know.