Services Module - 500 Internal Server Error When Accessing REST URL
The problem turned out to be the PHP version on the server.  The Rest Server had a line of code that requires PHP 5.3.  

There is an issue at https://drupal.org/node/2003396 and a patch at https://drupal.org/files/ServicesRESTServerFactory.inc_.patch

Installing the patch solved the problem without the need to upgrade the php version on the server