bgrabkowitz,
yes iframe works fine, but brings some issue with expand tags, the best will be a native way to have asynchronous way to show any tag, it seems that the guys from openx managed to do it :
http://docs.openx.com/ad_server/adtagguide_asynchjs.html
I tried DFP async option, but it's not suitable as it seems that the just work around the issue and create an iframe with their async tag and the issue of the expand tag happen too.
I am not sure if it's do-able, but as Google is now ranking site depending on their speed too, I guess this kind of option will be more and more asked by publishers.
Wanted to share my experience and open the discussion to see what is faisable around this point.
Cheers,