Great idea. If this gets wide adoption, which I hope it does, you'll want to ensure that your Javascript code is on a fast CDN or hosted locally, otherwise the sites will stall loading it.
I've got a lot of faith in varnish, so I'm pretty sure things should hold up on this end. However, yes, if you are worried about relying on an external resource by all means curl/wget/whatever the js file and server it up yourself. You just won't get any fixes or features we push in the meantime.
