Interacting with Iframes using Javascript

// reference to iframe with id 'ifrm'

var ifrm = document.getElementById('ifrm');

// reference to window in the iframe
var win = ifrm.contentWindow;

// reference to document in iframe
var doc = ifrm.contentDocument? ifrm.contentDocument: ifrm.contentWindow.document;

// reference to a form named 'ifrmTest' in iframe
var form = doc.getElementById('ifrmTest');

http://www.dyn-web.com/tutorials/iframes/refs.php