There are already solutions to this. The blogger uses Disqus for comment, which allows you to Sign in via Twitter and OpenID. If you use OpenID w/ AX or SREG

There are already solutions to this. The blogger uses Disqus for comment, which allows you to Sign in via Twitter and OpenID. If you use OpenID w/ AX or SREG you can get the users email address, which is even nicer. Done and done.