Bornakk confirmed it today. Or at least, he did not deny it:
Last I heard they would be removed when 3.2 comes out, if this changes we’ll let you know.
What’s that, you say? Specifically, he’s talking about the rewards for the meta-achievements Glory Of The Ulduar Raider and Heroic: Glory Of The Ulduar Raider.
I’m not entirely sure as to why they would do this. My guess is that they will be updating the achievement and want a mount that goes with the latest raid they add. I’m sure those that already have the mounts will be keeping them, Blizzard has never taken mounts from people before (think back to the original reskins of raptors, kodos, etc.).
I guess this implies that a new raid is coming in 3.2?
Zeke, This will be the same as the mounts for the Naxx “Glory of” achievements. That were removed when Ulduar was released with 3.1. They are being removed to make it so that they are special and so that not everyone has them.