postFormAsJson.js

function itWorked(response) { console.log("yay!"); }
function itFailed(err) { console.error("boo!", err); }

// in dojo 1.4+

var theForm = dojo.byId("theForm"); // assumes <form id="theForm"> is what you want

var def = dojo.xhrPost({
  url: "/send/it/here",
  rawBody: dojo.formToJson(theForm),
  contentType: "application/json"
});

def.addCallback(itWorked);
def.addErrback(itFailed);