The new Custom Menu is live on WordPress.com, arrived at via Appearance>Widgets>Custom Menu widget.
Don’t yet have a clue how to work with it and will certainly be spending lots of time experimenting over the next day or so on the test blog (so as to avoid accidentally pinging you all again). Hope by the time I wake up there will be a new addition to the Dashboard’s navbar and a little bit more information on how to work with the feature. Meanwhile from this side of the planet, it’s time to say goodnight!
Update: The Menus menu now appears under Appearance in your WP.com dashboard navbar. No official Support Doc yet but a link to the WP.ORG codex, which is also pretty scarce info-wise at the moment. Coffee first before experimenting.
Update June 2: Support Doc now available.