The Comment Permissions module enables control of commenting by user role and by node type. Additional user permissions for selected node types are added to the user access system so you can configure commenting with more control than Drupal core provides.
Note that "reply" links below comments may still appear for users without the permission to add comments. This is unfortunate, but I haven't found a workaround. See http://drupal.org/node/185855 for more information. Of course, the links can easily be removed in the theme layer.
The D7 update was done by darrick.