Hi,
You can use following script to do that. Please post your Development related questions in CRM Development forum to get the assistance at the earliest.
var iframe = document.getElementById('iframename');
var iframeContent = iframe.contentWindow.document;
var fieldinIFrame = iframeContent.getElementById('fieldname');
PS: This approach is not a supported way as this is not documented in SDK.
Hope it helps ...!!!
Vikranth
http://howto-mscrm.blogspot.com "Please Mark it as answer if it helps in resolving your query"