Theming the confirmation page

According to the THEMING.txt file included with Webform, you can customize the Webform's confirmation page by copying the webform-confirmation.tpl.php from the webform module directory into your theme's directory.

THEMING.TXT also suggests you can override a specific webform confirmation template by copying the default template to your theme directory with the name webform-confirmation-[nid].tpl.php

Reference: http://drupal.org/node/711534