Showing different sized ads depending on availability

I’ve been struggling with this for the better part of the week, and haven’t seemed to find a solution through the wordpress drums, so here it goes:

I would like to use 970×250 ads on my website (contentdope.com). This isn’t the problem, as it’s easy to create an ad through, say, adsense and just add it. The problem is, if the ad isn’t available, I would like to then use a passback that is more available — say, 728×90. Is there any way to do this? Help!