http://www.salon.com/2011/12/16/steidl_documentary_imprint/singleton/ Scroll down this page and the 2nd Adsense unit will float so it stays in view. Is this allowed or just for premium publishers? (most of the time I see Adsense there but sometimes other ad networks)
But I also have seen this on many sites with adsense? Why is it then not allowed? Whats a premium publisher?
You will find various plugins on WordPress to perform this task. Many blog owner thinks their visitor might get bore of seeing same advertisement to their visitors on their every visit so they use such plugins to shuffle 2-3 ad-networks ads and show the useful one.