In the horizontal format, it seems we're limited to 2 listings whereas in the vertical format, 4 listings show at once. There should be a way to display 4 listings in the horizontal format. Just a suggestion.
Some of the ad networks are running 728x90 "leaderboard" banners that contain four AdWords each. I can't believe that AdSense won't offer them in the future, since they offer higher revenue potential for Web sites that doan't have room for skyscrapers.