Here is an example that works on Chrome 5.0.375.125.
The page B (iframe content):
<html>
<head></head>
<body>
<script>
top.postMessage('hello', 'A');
</script>
</body>
</html>
Note the use of top.postMessage or parent.postMessage not window.postMessage here
The page A:
<html>
<head></head>
<body>
<iframe src="B"></iframe>
<script>
window.addEventListener( "message",
function (e) {
if(e.origin !== 'B'){ return; }
alert(e.data);
},
false);
</script>
</body>
</html>
A and B must be something like http://domain.com
EDIT:
From another question, it looks the domains(A and B here) must have a / for the postMessage to work properly.