(It's kinda-sorta live; players report attempting to purchase through the in-game store returns an error, and not all realms can 'see' it in that store. It seems the error is rectified. Also, despite the official forum thread announcing the mount, it isn't on the website version of the Blizzard Shop.)
The Iron Skyreaver is purchasable via the in-game browser. The item is deposited directly into your character's bags (unless your inventory is full, in which case you'll need to retrieve it from the mailbox). It seems that, for now, the Iron Skyreaver will remain an in-game shop item. Possibly to get players to use it.
The prices: USD 25.00 for North American region and EUR 20,00 for European region.
Official video (which at this time is still unlisted - another reason to believe the mount reveal was an "oops")