Comet implementation
As written in that answer, that is blocking a whole apache thread and therefore not a good idea.

I suggest you have a look at http://drupal.org/project/nodejs. node.js is an event-based webserver written in JavaScript and that is the module that integrates it with Drupal. Then, your clients keep a connection option with the node.js server and you can contact them through that server.

The project is still in development, I don't know how well it is working already.