req.body undefined in POST request in Express js

You have to use a bodyparser to process the body of a POST request

app.use(express.bodyParser());

and put this before the routing definition, like this

app.configure('development', function() {

 console.log("Development mode");

 app.configure(function() {
    app.use(express.bodyParser());
    app.use(app.router);
    .....
 });
});

here is the related google groups thread.