I just wanted to mention pmrpc - a JavaScript library for cross-domain inter-window RPC communication that uses the new HTML5 postMessage API for transport. The library provides a simple API for exposing and calling procedures from windows or iFrames on different domains, and also provides several advanced features like callbacks, ACL-based access control and automatic retries.
The main reason for developing pmrpc was the lack of easier to use and more advanced communication and access-control mechanisms in the postMessage API which supports only basic messaging.
Check out the project homepage for more information - http://code.google.com/p/pmrpc/ . Feedback and discussions are appreciated
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Cheers,
Ivan