demonstrating get_key function, which pulls session id out of query parameters if present, otherwise from cookie.

getkey-gist.js

// in app.js:

// ...
  app.use(express.bodyParser());
  app.use(express.cookieParser());
  app.use(express.session( { secret: 'verySecret', get_key: session.get_session_key }));
// ...

// in session.js (of my app):

exports.get_session_key = function(req,key) {
    if (req.param('session_id')) {
      return req.param('session_id');
    }
    if (req.cookies) {
      return req.cookies[key];
    }
}

yeah totally. although let's just make the option "key" or something, or at least camel-case like the rest of javascript