function randomLine(url,id){
new Ajax.Request(url,{
method: 'get',
onComplete: function(http){
var lines = http.responseText.replace(/[\n\r]\\n\r]?/g,"\n").split("\n");
$(id).innerHTML = lines[Math.floor(Math.random()*(lines.length-1))];
}
});
}

randomLine('http://www.eroero-funfun.com/ban/midban.txt','midban');