+1 for Beauty Pirate's recommendation.
I've got this installed as an available plugin on a multisite installation. I chose your plugin because it was the easiest for a client to configure. However, the two blogs I've got this running on already are getting several spams an hour from a single (US-based) bot.
I don't want to have my clients need to enable a second capcha plugin for this to work. It would be best to have the capcha functionality built-in, or to use the Growmap Anti Spam approach listed above, as this seems to be an effective, low-overhead strategy.
Thanks for a great plugin!