I love the Service Oriented Architecture. But like all things, security is needed. In this case to make sure that one service has permission to talk to another service. There are a few different ways to obtain this security but I really like using SSL certificates. It’s very simple to add other services and your webserver (apache, nginx) will handle the validation for you.
I wrote an article on getting this setup here: http://www.sitepoint.com/inter-service-communication-using-client-certificate-authentication/