I think we've all agreed that the technology of choice should should be restful. Since restful services return xml, i suppose we can say that XML is the data standard. Obviously, the web service should return relevant data for each service, what else would be useful for the client to return? For example, the login service would return a session key:
or on a failure:
Because most of the node names and attributes will be service-specific, like "session" is for the login service, I suppose the stuff that we can standardize is the root elements and associated attributes, maybe error nodes? What else?
<?xml stuff... > <osdash apiversion="0.1"> <session>23432ffaafddd32423dd2ff</session> </osdash>
<?xml stuff... > <osdash apiversion="0.1"> <error>bad username or password</error> </osdash>