The Celestial Steed mount and Lil' XT pet are now active and available for purchase at the Blizzard Store. Both the mount and the pet are account bound, meaning any current and future characters on a single account get them. Use the following links to get to the items now to buy them:
The mount will operate at whatever level of riding skill you've trained. This includes both ground mounts and flying mounts.
We now also have a blue post on the matter:
Nethaera -- Pet Store Additions