By Ben Vinegar and Anton Kovalyov, authors of Third-Party JavaScript
Legacy browsers—including Internet Explorer versions 6 and 7—have absolutely no support for the window.postMessage API so you hve to fall back to some other approach. In this article based on chapter 5 of Third-Party JavaScript, the authors discuss four hacks: using property, using window.hash property, using window.opener property, and using a Flash object.

window.postMessage Alternatives for Legacy Browsers (PDF)