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
We're pleased to be able to bring two new additions to the Blizzard Pet Store: the miniature robot Lil' XT and the Celestial Steed, the first mount to be available there. These fun options are available exclusively through the online Blizzard Store. [http://www.blizzard.com/store]

For more information, check out the Pet Store FAQ, which holds answers to most common questions: http://us.blizzard.com/support/article.xml?locale=en_US&articleId=29845

This article was originally published on WoW Insider.

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