app.js

/**
 * Module dependencies.
 */

var express = require('express')
  , routes = require('./routes')
  , user = require('./routes/user')
  , http = require('http')
  , path = require('path');

// XXX Comment this line out and everything works fine
var express_nm = require('express-namespace');

var app = express();

app.configure(function(){
  app.set('port', process.env.PORT || 3000);
  app.set('views', __dirname + '/views');
  app.set('view engine', 'jade');
  app.use(express.favicon());
  app.use(express.logger('dev'));
  app.use(express.bodyParser());
  app.use(express.methodOverride());
  app.use(app.router);
  app.use(express.static(path.join(__dirname, 'public')));
});

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

function some_mid(req, res, next) {
  console.log('middleware called');
  next();
}

app.param('user_id', function(req, res, next, id) {
  console.trace();
  console.log('app.param user_id called', id);
  if (req.user)
    console.log('req.user already set to', req.user);
  req.user = id;
  next();
});

app.get('/user/:user_id', some_mid, function(req, res) {
  res.send('/user' + req.user);
});

app.get('/', routes.index);
app.get('/users', user.list);

http.createServer(app).listen(app.get('port'), function(){
  console.log("Express server listening on port " + app.get('port'));
});

app.param('user_id') is called twice comment line #13 and it's fixed