jimh009 - 9:52 am on May 19, 2013 (gmt 0)
I've tested it by narrowing the browsers window width (in Firefox and IE10) and also on my ipad and iTouch. Results are the same in all.
I haven't set on the final sizes where different ads will be displayed. Most likely I'll use 3 different sizes for the reason you mentioned. But for testing purposes, figured i'd start with two different ad sizes just to get things working.