As Nethaera and Eyonix have both pointed out on the forums before, the Phoenix mount is indeed in game. But as I had the honor to learn recently, it's not in the place that many of us might think it is. At least, the first one isn't. The others -- well, nobody but Blue has the vaguest idea where those are going to drop. (Of course, most people figure on Kael'thas or Al'ar in Tempest Keep as the drop source for the Phoenix Mount, much like the tiger mount drops in ZG from the tiger boss, and the Baron drops his own mount in Stratholme.)
You see, the World First phoenix mount belongs to Ezra Chatterton, the young man who recently went to Blizzard's offices with the Make-A-Wish foundation. Ezra was given the first ever Phoenix mount in game for two reasons. First, it was probably the best gift that Blizzard could really give any gamer as far as one of the new Outland mounts. Netherdrakes are awesome, but a phoenix trumps that, if you ask me! And hey, who wouldn't want a mount that could move at 310% speed like the Swift Nether Drake arena award mount. Secondly, as Ezra's father Micah explained to me, the phoenix is an extremely important personal symbol for Ezra.