It's a scheme relative URL. So if your page is https, then the js/css request will also be https. Works everywhere, I believe, except that I've read IE8 will issue the request twice, which would be detrimental to performance since the number of HTTP requests is one of the most significant factors. But if IE8 isn't a concern, then there's no reason you can't use scheme relative URLs everywhere.
EDIT: People keep beating me to the punch lately.