Having a chat frame in your web page is really easy if you use WebSockets and can run a simple chat engine on your server (some hosting services don't allow sockets at all)
Since WebSocket is not available everywhere yet, use a Flash interface (pure actionscript, no visuals) with the same methods: onopen, onsend, onreceive, onclose, to communicate with the server. So when WebSockets is available all you have to do is replace the flash object with the real stuff.
mychat = new WebSocket(chatserverurl);
mychat.send("Hello");
mychat.send("Can you hear me?");
mychat.send("Am I getting through to you");