var l = (screen.width-280)/2; 
var t = (screen.height-430)/2;

function comenta(postid) {	
  window.open("http://www.samila.com.br/ricardo/ieufala/comentarios.php?postid="+postid, "", "height=370,width=280,left="+l+",top="+t+",scrollbars=1");
}