Yes, it is getting a little more difficult to join the Google AdSense program. I do believe that at the beginning of last year they added more rules you must pass in order to be eligible to join AdSence.
One method of displaying advertisement that I am not 100% sure of is the (Background Opaque 50% etc.. when the user hovers over ad) in order to make the ad stand out. Seems like a good idea but like I said not to sure. First noticed this method on DaniWeb.