I love being able to use Webform to quickly whip up a form for a client, like for a web-to-lead page or a contacts database. One customization that clients almost always request is to modify the text on Webform's confirmation page. Unfortunately the way to do this isn't immediately apparent... in fact it's kind of hidden!
According to this post on Drupal.org, all Webform response text should be configurable. There is a special template you can use to configure this text.
The template file is actually inside the Webform module's home directory, and it's called webform-confirmation.tpl.php. Here's the default code:
* Customize confirmation screen after successful submission.
* This file may be renamed "webform-confirmation-[nid].tpl.php" to target a
* specific webform e-mail on your site. Or you can leave it
* "webform-confirmation.tpl.php" to affect all webform confirmations on your
* Available variables:
* - $node: The node object for this webform.
* - $confirmation_message: The confirmation message input by the webform author.
* - $sid: The unique submission ID of this submission.