Real-time bi-directional JSON-RPC communication over HTTP

I think you should have a look at socket.io to accomplish your task. You could if you wanted to watch this video from the author: "Socket.IO Workshop: Guillermo Rauch". It is easy to work with on both server as client. I have created a simple sample pubsub using redis on top of socket.io.

You should take a look at JSONRPC-bidirectional.It supports bidirectional RPC over WebSocket, Worker, WebRTC and HTTP and it is highly extensible.

To my knowledge, Streaming is supported by FF, Chrome (Has bufffering issues that require a datatype of application/octet-stream or a prelude to work) and IE8 (through a little XDomainRequest). I don't know about opera.

I don't really know of any comet industry standards, the Bayeux is probably the closest. It's hard to see how facebook/gmail/twitter do it as all the code is obfuscated, and it's exceedingly difficult to find much info on how all the browsers handle everything.

Even more difficult is that you will need to use a specialized server, keeping this many connections open will require thread pooling etc.. A normal server will blow up pretty fast.

It is a very powerful design if you can get it to work reliably though.