My RBL-checking anti-spam plugin for WordPress seems to be working fine, and is catching about 90% of my comment spam (and I’m getting a lot today).
Here it is then, for you to try out.
Save that file somewhere, rename to rbl.php, and then read the file for setup instructions. BE CAREFUL! This plugin will DELETE COMMENTS based on the content of RBLs. Don’t choose RBLs that list dialup space, for example. You break it, you bought it. ;)
Let me know how it works out for you.

Hey – uh, this whole “auto delete” thing doesn’t seem like such a good idea.
How about a “review” screen or something? When it just lists comments in moderation with check boxes all checked. You can quickly scroll through and uncheck any false negatives (good, real posts) and then hit, “Delete the junk”.
This is definitely an option – it would be easy enough to make it tag comments for moderation and then you can just use the WordPress moderation tools as normal.
However I wrote this initially to do comment deletion because my moderation queue is already full of spam posts (I got 40 overnight last night) and I’m getting bored of deleting them :P
Look for a require-moderation-mode option in the next version of the script :)
List of all WordPress Antispam Plugins.