I know there were a bunch of folks waiting on this, so I'm happy to say the REST API documentation for the HPC Pack 2008 R2 SP3 release is available.
Windows HPC Server 2008 R2 with Service Pack 2 (SP2) provides access to the HPC Job Scheduler Service by using an HTTP web service that is based on the representational state transfer (REST) model. You can use this REST API to create client applications that users can use to define, submit, modify, list, view, requeue, and cancel jobs.
Windows HPC Server 2008 R2 with Service Pack 3(SP3) expands the HTTP web service to provide additional operations that provide information about nodes, node groups, and the cluster name. Windows HPC Server 2008 R2 with SP3 also provides operations that allow you to create and manage SOA sessions, and to send SOA requests and receive SOA responses for these sessions. The SOA operations that are available only when the REST web service is hosted in Windows Azure. All other operations are available either when the REST web service is hosted on an on-premise cluster or when the REST web service is hosted in Windows Azure.