JQuery/AJAX registration

What do you think about this?

var Request = {
    version: 1.0, //not needed but i like versioning things
    xproxy: function(type, url, data, callback, timeout, headers, contentType) 
    {
        if (!timeout || timeout <= 0) { timeout = 15000; }
        $.ajax(
        {
            url: url,
            type: type,
            data: data,
            timeout: timeout,
            contentType: contentType,
            success:function(data) 
            {
                if (callback != undefined) { callback(data); }
            },
            error:function(data) 
            {
                if (callback != undefined) { callback(data); }
            }, 
            beforeSend: function(xhr)
            {
                //headers is a list with two items
                if(headers)
                {
                    xhr.setRequestHeader('secret-key', headers[0]);
                    xhr.setRequestHeader('api-key', headers[1]);
                }
            }
        });
    }
};

<script type="text/javascript">
var contentType = "applicaiton/json";
var url = "http://api.lastfm.com/get/data/";
var timeout = 1000*5; //five seconds
var requestType = "POST"; //GET, POST, DELETE, PUT

var header = [];
header.push("unique-guid");
header.push("23903820983");

var data = "{\"username\":\"james\"}"; //you should really deserialize this w/ a function
function callback(data)
{
   //do logic here
}
Request.xproxy(requestType, url, data, callback, timeout, header, contentType);
</script>