From this topic at Area51

http://area51.phpbb.com/phpBB/viewtopic.php?f=81&t=41673

A request has been made to give users the option to choose if jQuery should be hosted either by a local copy from their server, or from a remote CDN. This would allow the forum admins to decide which method they think is best for their board.

There should also be a fallback method in case the CDN option is selected, but fails.