I installed Jay Allen's MT-Blacklist plugin today. Lately, I've been getting a few comment spams a week and it seemed like now was as good a time as any to install MT-Blacklist before it got any worse. I did not have any trouble with the installation, but I won't know if it's really working until I get another comment spam.
The current blacklist now contains over 600 spam strings for immediate protection on install. Comment spammers, beware! I've added an "MT-Blacklist" icon to my logos to give credit to Jay, and to discourage any spammers who might be paying attention.