Thanks to Patrick Strang who pointed me to Steven Geen's simple anti-comment spam measure for MovableType, I managed to stem the current flow of "Please approve this comment" emails flooding my inbox. This has been happening since last Saturday! Argh! Why are they doing this even when everything goes into the moderation queue?
Anyway, it's so simple to get this into WordPress - just edit wp-comments.php and wp-comments-post.php to add the field to the comment form (see below) and die() when the correct "letter of the day" isn't entered. (Though die()ing isn't the most elegant way, but WordPress does this for the other fields as well.)
Check out the comment form if the picture above is too small. Sorry to have to put you commentors through this, but it's really for my sanity. At least it isn't one of those randomly-generated graphical thingies that really ensures you are human (or an equivalent intelligent lifeform).