Quick note to self in case I (inevitably) forget later. If you are using Apache mod_rewrite to make nice, clean URLs, and are also supporting JSONP, you may run into the situation where you have code that wants to append "?callback=xxx" to your URL (e.g., a cross-domain AJAX call in jQuery). Imagine you have a nice clean URL /user/123, which actually corresponds to user.php?id=123.