drupal/profiles/default/default.profile

<?php
// $Id$

/**
 * Implement hook_form_alter().
 *
 * Allows the profile to alter the site-configuration form. This is
 * called through custom invocation, so $form_state is not populated.
 */
function default_form_alter(&$form, $form_state, $form_id) {
  if ($form_id == 'install_configure') {
    // Set default for site name field.
    $form['site_information']['site_name']['#default_value'] = $_SERVER['SERVER_NAME'];
  }
}