My guess, though, is that we'll soon see an extension or option in browsers that will allow you to choose whether a source preloads or not (I would think kind of like how Chrome's Click to Play for plugins works). My understanding of HTML5 leads me to believe that if the web designer has it set to preload, there's nothing that you can do to tell it not to. The default location is relative to your current page in a folder named 'js'. The Jplayer.swf file must be uploaded to your server.
This file is used in browsers that do not support HTML5