This looks good, but to be more user friendly please move the callback name generation inside method "req".
So in result making the request will look like:

```javascript
jsonp.req('some/url', {data:'obj'}, 'callbackNameParameter', callbackFunction)
```

Example:
```javascript
jsonp.req('http://api.flickr.com/services/feeds/photos_public.gne', {format: 'json'}, 'jsoncallback', function(response) {console.log(response)});
```