It is a protocol relative url. This way if you are looking at the site over https then the assets will also come down over https.
As a bit of a general question to everyone, did old browsers not support it or something? Or do people prefer using relative urls over absolute style ones?
Also, for those pesky secured sites that have unsecured static assets on a separate server, it might be that those servers aren't configured for SSL.
There is an edgecase bug in IE6 that causes a dialog to blow up… under some security settings (unsure if they are default) when requesting form the non-'ssl' subdomain
So looks as if it's fine to use if you're not supporting IE6.
The new GA snippet uses different domains for the two modes so it can't be used.