well it seems i found my own answer:
Browser restrictions on cross-domain scripting in Iframes are necessary to prevent malicious data access. However, there are cases, such as third party served ads, where publishers would like to grant access to specific domains that they trust to leave the Iframe environment.
In order to enable this access, a publisher has to put a special file on their domain that acts as a bridge between the ad server domain and publisher domain. DART Motif requires that the publisher put a file called "DARTIframe.html" on a specific relative URL /doubleclick at the root directory of the web service on their webservers. This file can be downloaded from this article by clicking on the link to the right.
It seems like other providers of expandable ads (pointroll, eyeblaster) have similar techniques for overcoming the iframe problem. They'll ask you to put a file on your domain as well, and then let them know where it is.