This is Nick Riggs' excellent "Simple AJAX Polling Plugin for jQuery".
The original article and draft source can be found here: http://www.nickriggs.com/posts/simple-ajax-polling-plugin-for-jquery/
This code is assumed to be public domain, with the article Copyright © 2009 Nick Riggs
Nick Riggs <@nickriggs> [http://www.nickriggs.com]