That was nicely explained. I just want to add that there are some other cool plugins are out which help you to lower the amount of spam you receive, namely, http:BL WP Plugin.
This plugin checks the header information which is sent by your blog visitors, if it detects something which points to a bad application (Spambot, harvester and so on) it blocks it as a suspicious request. It stops alot more than just Spambots.

http:BL WP Plugin This Plugin needs an account at ProjectHoneyPot(PHP) in order to work(it’s free so why don’t you join?). This scans the IP address against a database with already known Spammer-, Harvester- and Dictionary-attacker- IPs and block them. You have a number of options to define behavior. If you add another account you could actually use this service as a Honeypot too.

Phil, thanks for the heads up, looks like a cool project.