banners = new Array(2);

banners[0] = "http://www.adbanner.com.br/conteudo/728x90/adsense.jsp";
banners[1] = "http://www.adbanner.com.br/conteudo/728x90/buscape.jsp";

var ran_unrounded=Math.random()*banners.length;
ran_number = Math.floor(ran_unrounded);

document.write('<iframe src="' + banners[ran_number] + '" width=728 height=90 scrolling=no frameborder=0 marginwidth=0 marginheight=0></iframe>');