[ozzon91]

вот так всеравно undefined

function vote(url, callback) {
	var req = createXmlHttpRequestObject();
	var result;
	
	req.onreadystatechange = function() {  
	if (req.readyState == 4 && req.status == 200) {
		result = req.responseText;
		if(typeof callback === 'function'){
			callback(result);
			}
		}
	}
	req.open("GET", url, true);
	req.send(null);
}

alert(vote(url, function(result){return result;}));