AdMob now offers mobile ads for non-mobile sites.
It's called the Adaptive Mobile Ad Unit. The ad automatically scales, even when users scale to view desktop versions of websites on their mobile browsers.
The ad appears - and stays - at the top of the page. Initially, this is only being rolled out for iPhone traffic.
A demo of the ad can be found by visiting TapTap.net on the mobile Safari browser on an iPhone. Here's a screenshot:
Download the code by signing into your AdMob account, creating a new site and selecting "iPhone Web."