After receiving a flurry of trackback spam (some from the same source as often as 3x every 5 minutes), I upgraded the site from Movable Type 3.31 to 3.35, as well as tweaked SpamLookup as well as added Akismet functionality, and so far (6 hours), all of the trackback spam has been caught. Hopefully the new layers of spam filtering are a long term success.