[eluser]Nick Husher[/eluser]
You could use a Javascript ping script that sends a request at a particular url on your server every so often. The ping script sets a "last heard from" flag on the server and returns all the text messages between the current time and the previous "last heard from" date. This way you could send messages at people who aren't actually connected to the chat server.
It's impossible without hackery (or HTML5, or a Flash/Silverlight plugin) to "push" data from the server to a set of known clients. And because HTTP requests are mostly-atomic, it's difficult to set up a real time connection with a browser between a client and the server. So the best you're going to get is a "turn-based" simulation of that connection.