In previous years, today would have been celebrated as World IPv6 Day. Although a little late to the party, as of this afternoon, npmjs.cf's edge is configured for IPv6. So, if you're on an IPv6 connection, you would have automatically started using it.
The npmjs.cf mirror is up and running, and ready to start taking requests. However, for the time being I'm calling this a “soft launch” as there's a few changes and tweaks that need to be done as I scale everything up.
I look forward to announcing a full launch in the next little bit.