It's really dependant upon the browser you're targetting, but in IE, you can reference the parent window with "window.parent".
What I did was have a form on the parent window that had several fields. When the value of the field changed I change the URL of the IFrame to pass a querystring of the field and value to be validated against some DB info. The page in the IFrame queries the DB, validates the value (or doesn't), then writes some javascript to change the color or background of the field on the parent window accordingly. So the JavaScript on the IFrame used "window.parent" to make those changes.
Example:
Code:
window.parent.document.myForm.phonenum.style.color = '#FF0000';
This would change the color of the text in the field "phonenum" on the parent window to red from the IFrame.