Best practices for building a bidirectional web applications?

I'm building a site that needs to communicate bidirectionally with both an iPhone client as well as a browser client.

Previously I've used RabbitMQ to do Comet style stuff. Ideally I'd like to use WebSockets, but as far as I can tell there isn't a backwards compatibility lib that will fall back to using Comet.

What's the "state of the art" as it were?