app.js

/**
 * Module dependencies.
 */

var express = require('express');
var rpcMethods = require('./methods.js');

var app = module.exports = express.createServer();

// Configuration

app.configure(function(){
  app.use(express.bodyParser());
  app.use(express.methodOverride());
  app.use(app.router);
  app.use(express.static(__dirname + '/public'));
});

app.configure('development', function(){
  app.use(express.errorHandler({ dumpExceptions: true, showStack: true })); 
});

app.configure('production', function(){
  app.use(express.errorHandler()); 
});

// Routes
app.get('/', function(req, res){
  res.redirect('test.html');
});

// RPC end point
app.post('/rpc', function(req, res) {
  res.header('Content-Type', 'application/json');

  // Add in support for GET requests
  if(req.method === 'GET'){
  	var url_parts = url.parse(req.url, true);
  	var data = req.query;
  }
  else if(req.method === 'POST'){
  	var data = req.body; 
  }
  else{
  	onError({
      code: -32603,
      message: 'Exception at method call',
      data: e
    }, 500);
  }
  

  var data = req.body, err = null, rpcMethod;

  if (!err && data.jsonrpc !== '2.0') {
    onError({
      code: -32600,
      message: 'Bad Request. JSON RPC version is invalid or missing',
      data: null
    }, 400);
    return;
  }

  if (!err && !(rpcMethod = rpcMethods[data.method])) {
    onError({
      code: -32601,
      message: 'Method not found : ' + data.method
    }, 404);
    return;
  }

  try {
    rpcMethod(data.params, {
      onSuccess: function(result) {
        var resString = JSON.stringify(
        {
       	    jsonrpc: '2.0',
            result: result,
            error : null,
            id: data.id
        });
      	//handle JSONp - add this around result string
      	if ( req.query && req.query.callback ) {
	    resString = req.query.callback + '(' + resString + ');';
        }
        res.send(resString, 200);
      },
      onFailure: function(error) {
        onError({
          code: -32603,
          message: 'Failed',
          data: error
        }, 500);
      }
    });
  } catch (e) {
    onError({
      code: -32603,
      message: 'Excaption at method call',
      data: e
    }, 500);
  }
  return;

  function onError(err, statusCode) {
    res.send(JSON.stringify({
      jsonrpc: '2.0',
      error: err,
      id: data.id
    }), statusCode);
  }
});

// Only listen on $ node app.js
if (!module.parent) {
  app.listen(3000);
  console.log("Express server listening on port %d", app.address().port)
}

Added GET/JSONp support to hagino3000's nice json-rcp route.