<i>For example, what is the correct status code to send when a new resource has been created?</i><p>From <a href="http://en.wikipedia.org/wiki/List_of_HTTP_status_codes" rel="nofollow">http://en.wikipedia.org/wiki/List_of_HTTP_status_codes</a><p>201 Created
The request has been fulfilled and resulted in a new resource being created.<p>This isn't to say you shouldn't document how your service responds to requests, just that there are response codes you can use that are widely understood.<p>If you want you can respond to everything with 200 OK and require clients to parse responses to understand how your service handled a request, but again this is the same thing XML-RPC gives you.