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.
+> I hate CAPTCHAs with a passion. Someone else is welcome to write such a
+> plugin.
+>
+> If spam via openid (which I have never ever seen yet) becomes
+> a problem, a provider whitelist/blacklist seems like a much nicer
+> solution than a CAPTCHA. --[[Joey]]