express.js sessions with reverse proxy

I've setup an Apache reverse proxy to forward api.mydomain.com to localhost:2000 which works just fine.

However, the real issue i'm having is in regard to sessions - it seems that the req.session's aren't being stored when querying via api.mydomain.com. Sessions will work just fine visiting from localhost:2000.

I assume this has something to do with the proxied domain...

server.js

var express = require('express'),
    app = express();

app.enable('trust proxy');

app.configure(function() {
    app.use(express.bodyParser());
    app.use(express.methodOverride());
    app.use(express.cookieParser());
    app.use(express.session({ secret: 'supersecret'});
    app.use(app.router);
});

app.get('/session', function(req, res, next){ 
    console.log(req.session.username)
    // this will be undefined when called from  api.mydomain.com...
});


app.post('/session', function(req, res, next){ 
    req.session.username = 'my username';
});

apache config

<VirtualHost *:80>
    ServerName api.mydomain.com   
    ProxyPass / http://localhost:2000/
    ProxyPassReverse / http://localhost:2000/

    ProxyPreserveHost On
    ProxyPassReverseCookieDomain api.domain.com localhost:2000
    ProxyPassReverseCookiePath / /
</VirtualHost>

EDIT

Notably, for further testing - adding the below code before app.use(app.router) in app.configure() ....

app.use(function (req, res) {
    res.send('<h2>Hello, your session id is ' + req.sessionID + '</h2>');
});

will result in the following (each line represents a new GET /session request to the app - node hasn't restarted either).

localhost:2000

Hello, your session id is sHkkESxJgzOyDhDwyTjwpNzq
Hello, your session id is sHkkESxJgzOyDhDwyTjwpNzq
Hello, your session id is sHkkESxJgzOyDhDwyTjwpNzq

api.mydomain.com

Hello, your session id is uuo4U5ierZAG8LSH1BdwTlVf
Hello, your session id is 8BxL97Bo35SDt4uliuPgnbia
Hello, your session id is 0xkqZZpzQNvTsQpbJtUlXgkR

Setup info

NodeJS v0.8.8

Apache v2.4.2

ExpressJS v3.0.0rc4

UPDATE 2

Well, I've so far resolved to simple use mydomain.com/api, as that seems to fix everything. So I suppose it's just something to do with how Apache handles domain communication with Express!