Today I needed to hide the masterpage in modal popup that was showing a SharePoint layouts page. I used a redirect within the modal popup from one page to another. This redirect made the masterpage appear in de modal, which I didn’t want.
To make sure the masterpage is not showed the URL paramter (querystring) “isDlg=1” should be added.
So for instance an possible URL is: http://www.myurl.nl/somepage.aspx?isDlg=1.