Multiple Domains using one WP install From http://wordpress.org/support/topic/multiple-domains-using-one-wp-install-not-wpmu

wp-multidomain.php

<?php
/* Determining the domain  - DO NOT EDIT */

$host = $HTTP_HOST;
$parts = explode('.',$host);
if ($parts[3] = "") {
    $domain = $parts[0];
} else {

    $domain = $parts[1];
}
/* Domain - Edit below. Add more "cases" for each domain you want to have.
   Make sure each "case" has a "break; line at the end.                */

switch ($domain) {
  case "domain1":		// "domain" in "www.domain.com"
    $db = "database1";		// the database for this domain
    $user = "username1";	// the username for this database
    $password = "pass1";	// the password for this database
    $hostname = "localhost";	// 99% chance you won't need to change this value
    $table_prefix  = 'wp_';	// change for multiple installations in one database
    $wplang = '';		// change to localize wordpress (must have an MO file in wp-includes/languages)
    break;

  case "domain2":		// "domain" in "www.domain.com"
    $db = "database2";		// the database for this domain
    $user = "username2";	// the username for this database
    $password = "pass2";	// the password for this database
    $hostname = "localhost";	// 99% chance you won't need to change this value
    $table_prefix  = 'wp_';	// change for multiple installations in one database
    $wplang = '';		// change to localize wordpress (must have an MO file in wp-includes/languages)
    break;
}

/* That's all, stop editing! Happy multi-blogging. */

define('DB_NAME', $db);
define('DB_USER', $user);
define('DB_PASSWORD', $password);
define('DB_HOST', $hostname);

define('DB_CHARSET', 'utf8');
define('DB_COLLATE', '');
define ('WPLANG', $wplang);

define('ABSPATH', dirname(__FILE__).'/');
require_once(ABSPATH.'wp-settings.php');

?>