This Wordpress plugin allows you to change the 'Leave a Relpy' message that appears as the heading of the comments form.
You can specify the message to be shown on a per-post basis. The following screen-shot shows how the user interface looks.
If you do not specify a comments section title for a certain post, it will default to the global value. You can set this global value on an admin panel. The following screen-shot shows this.
If the plugin does not work, check to see if the comments.php file of the currently activated theme calls the 'comment_form()' function.
This plugin should work from Wordpress version 3.0 upwards and has been tested on 3.0, 3.1.1 and 3.1.2.
In order to install the plugin, download it from here and upload the contents of the zip file to the wp-content/plugins folder. Then activate the plugin. You can also install it from the Wordpress admin dashboard.