Subspace: Secure Cross-Domain Communication for Web Mashups
Check out this research paper presented at WWW 2007
http://www2007.org/papers/paper801.pdf
This paper shows the technique of nested iframes, to enable secure cross-domain communication. The technique relies on document.domain, and offers details on iframe security in the various browsers.
I tried Googling for that javascript lirbary to no avail. I have emailed the authors and am waiting for a reply.
There is enough detailed information in the paper to code this library in javascript. If I don't hear back from the author's I might get started coding it myself because its a really awesome technique for secure cross-domain communication using iframes
Benjamin