Unfortunately we have discovered a syntax problem in the 4.2.3 upgrade script which will cause the upgrade to fail under certain circumstances. We are pulling this release and will replace it with 4.2.4 shortly.
If you've downloaded 4.2.3 and not tried the upgrade yet, you can trash that tarball and then download 4.2.4 shortly.
If you tried to upgrade using 4.2.3 and got the fatal error, you can safely download and run 4.2.4 shortly.
This issue will not affect your site if you installed a new CiviCRM site using 4.2.3.