There is an option in WordPress Settings->Discussion that allows forcing users to register and log in to comment in your pages or posts. This plugin extends that option individually to each page or post, allowing you to force login only in the post or pages that you want.
In the same way but in the opposite case, if you are already forcing users to login you can open the comments on any post or page.
Make sure that you use get_option('comment_registration') or comment_form() on your theme’s comments-template to make this plugin works.
Comments, questions, feature requests and bug reports are welcome: http://www.berriart.com/en/comment-privileges-by-post/