I don't necessarily trust all OpenID providers to stop bots.  I note that ikiwiki allows [[banned_users]], and that there are other todos such as [[todo/openid_user_filtering]] that would extend this.  However, it might be nice to have a CAPTCHA system.

I imagine a plugin that modifies the login screen to use <http://recaptcha.net/>.  You would then be required to fill in the captcha as well as log in in the normal way.