Whenever the content of the parent and (i)frame is loaded from a host with the same hostname then you are OK, if you have the ability to set the document.domain property in the documents loaded.

Caveat: they have to be set in BOTH documents, i.e. NOT only the iframe content.

Then you can easily access content on http://www.example.com from whatever.example.com and vice versa.

(in this example you’d have to set:

document.domain = “example.com”;

in BOTH, the accessing and the accessed document)