I am currently trying to set up a basic authentication system for my node.js app. For now I am using express (3.0.0rc5), passport (0.1.12) and socket.io (0.9.10) with Mongoo
You can make use of express-socket.io-session. You can get the passport session user with socket.handshake.session.passport.user . It supports both reading and writing to the session store.
NB: Works with express > 4.0.0 and socket.io > 1.0.0 and won't be backward compatible