While it remains to be seen if this works, I've just come across (well, more of a deliberate search) what seems to be a reasonable way to reduce comment spam. It adds a quick maths (not "math" as the plug-in author erroneously suggests) question to the Comments section which must be correctly answered in order for the comment to be posted. Akismet seemed reasonably good at catching spam, but I'm hoping this should properly keep the comments section clean.
Head here to download it (comes with full setup instructions). Also yes, I am tempted to "sed s/math/maths/" the source. :P Future edit: plugin site's gone