With Ubercart, where is the $email_text variable set, the variable that determines whether the "do not reply to this email" should be displayed in the email received by a customer after they place an order.
It's set in
template_preprocess_uc_order(&variables) in the order module (
If you're looking to override this variable for your site it can be overridden by defining a function
MYTHEME_preprocess_uc_order(&variables) in your theme's template.php