When my GF pre-ordered Pandaria, they sent her the mount and the pet right away, but when Pandaria was released, she couldn't download the client.
Turns out, something went wrong with the payment (on their end), so they stripped the mount/pet from her account and made her buy the expansion again after release. Though she still has the banner/sigil in D3 for the pre-release purchase, she had to continue without the pet and mount.
Don't get your hopes up.
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