The SMF forum software has a better version of Captcha than the old phpBB forum did, and we're using it at 'highest strength' available. We also require the user respond to the activation email (to validate email address) and if an account hasn't been activated in a week or so, we remove them.
So.. yes, we've had more members sign up, some may have been spambots (but haven't spammed yet), but most have been real people .. just what we'd wanted
The admins get an email for every member signup, so we're keeping close tabs on what's happening with registrations.
Thanks for your question.. hope this helped explain what we're doing.