I have a site on A.com and an iframe on B.com which calls javascript from A.com. This works great in FF. In IE7 / 8 I am getting a
Message: Access is denied
If both the pages are under your control (i.e. they belong to you and you cn alter the code), try this -
http://www.tomhoppe.com/index.php/2008/03/cross-sub-domain-javascript-ajax-iframe-etc/
Set document.domain like this:
<script type="text/javascript">
document.domain = 'tomhoppe.com';
</script>
IFrames can communicate as long as they are "of the same origin" - so same domain and same protocol. Communication is blocked if they are of different origin.
HTML5 introduces a new communication mechanism. It may be worth looking at http://www.w3.org/TR/webmessaging/
http://en.wikipedia.org/wiki/Cross-document_messaging
It is also worth reading up on the security implications
https://www.owasp.org/index.php/HTML5_Security_Cheat_Sheet#Web_Messaging