Gravatar Signup makes it really easy for your commenters to sign up for a Gravatar. All they have to do is check a box, fill in a desired password, and when they submit their comment to your site, they'll be signed up for Gravatar.
With WordPress 1.5's default templates, it requires NO hacking of the comment form. Just drop it in and activate!
To see it in action, as well as download it: