I would highly highly recommend checking out the APE project. It stands for Ajax Push Engine and it uses Comet Server techniques/technology. This project is designed to handle tens of thousands if not hundreds of thousands of users at a time and it provides the server end and the JS interface client. It is compatible with all the major JS libraries.
Its well thought out, clean, and most importantly FREE!
Also I am sure that there are CMS plugins that exist that utilize it. The DrupalChat module has been talking about using it.
