A really simple example ...
<iframe id="fred" width="200" height="200"></iframe>
var s = document.getElementById('fred'); s.contentDocument.write("fred rules");
the "contentDocument" is the equivalent of the "document" you get in the main window, so you can make calls against this to set the body, head, any elements inside ... etc.
I've only tested this in IE8, Chrome and Firefox ... so you may want to test in IE6/7 if you have copies available.