Just stumbled upon a requirement to use REST for an EU software project;
System to system interoperability Should use: 1. Roy Fielding's REST principles
Of course, in the same list they also mention “XML-RPC”, which cannot be used RESTfully (unlike SOAP, also in the list), but it’s a start. I’ve talked with folks from other government projects, and they wanted to use REST because they wanted to use the principles that made the Web work; they just weren’t exactly sure how to go about it.