You can never have enough DATA....
I have gradually built up a solid (100%) defense against trackback spam in my WordPress 2.3.3 blog which I host at home (Mac OS X Server). The ingredients are as follows:
-- Registration is required, with user's email address confirmed.
-- Trackbacks are OFF in WordPress settings (as if that makes any difference).
-- Akismet 2.1.4 plugin installed and activated.
-- Worst Offenders 0.2a plugin activated.
-- WP-Ban 1.20 plugin activated.
-- Bad Behavior 2.0.13 plugin activated.
Akismet definitely "works" to the extent that it thoroughly filters trackbacks and allows you approve/disapprove stuff. But it's annoying and tedious, especially because I have trackbacks already turned off in WP! You can always simply let Akismet age and then trash trackbacks automatically.
Worst Offenders and WP-Ban work well together, if you enjoy tinkering. Worst Offenders clusters the main sources of trackback spam by sender's IP#, and if you collect all those IPs and add them to WP-Ban's banned-senders list, you can eventually lock out in advance 99% of the trackbacks that Akismet would normally collect. You only need to update WP-Ban's banned list occasionally thereafter. Plus WP-Ban lets you display any rude error-message HTML page to spammers. :-)
If you want to KILL trackback spam entirely, I suggest the Bad Behavior plugin. It hasn't malfunctioned ("mal-WHAT?") for me at all, even while it's running concurrently with the other plugins listed above, and I suspect that I could run only Bad Behavior and get 100% protection (I may try that later). Admittedly this very powerful plugin needs careful observation, might block the wrong trackbacks or users, or might conflict with other plugins. Drive carefully. Byee!