As you examine the specification, you’ll probably notice that it looks like the?ArcGIS REST API. This is deliberate. The pattern we have used at Esri for exposing REST-ful GIS services has been embraced by thousands of developers who use the ArcGIS Web APIs. It is a simple and intuitive way of structuring and talking to GIS Web services. We wanted you to feel free to implement services that follow the same pattern.
Whether or not this is truly an open spec (and not opening the debate as to how “RESTful” this spec really is), the rush begins for everyone to implement this spec on their own apps so they can be used with ESRI clients.
Jack opens his secret to getting RESTful with ArcGIS