Dan,

This site is D6, so things work differently than is described in this post. D6 has some strange behaviors that allows us to change the #value in the theme layer and still let the user change those fields. In D7 you would want to use #default_value.

Example:

$form['your_comment']['subject']['#default_value'] = "Enter a subject";

You may want to also check out this resource for other form api references:

https://api.drupal.org/api/drupal/developer--topics--forms_api_reference.html