When patch 3.1 came out, the Plagued and Black Proto-Drakes, which were rewarded for the normal and heroic tier 7 raid meta-achievements, respectively, were removed from the game. You can still do the achievement, but it doesn't give you a drake any more. The thinking behind this was that people would be able to come back with Ulduar gear and complete the achievements (relatively) easily, and Blizz wanted the accomplishments of those who had done the achievement pre-Ulduar to stand out.
It looks like the Rusted and Ironbound Proto-Drakes are going to receive the same treatment in patch 3.2: they will no longer be attainable. These are the drakes that are rewarded for the Ulduar raid meta-achievements, on normal and heroic. This is interesting because it implies that we'll get a new tier of gear in patch 3.2. Information about what the expansion of the Argent Tourney is going to bring PvE-wise is still pretty scarce; we know there will be some 5-, 10-, and 25-man content, but we're not really sure what it'll be like yet. All we've heard so far is "Think Ring of Blood or Amphitheater of Anguish, but more epic." Here's hoping for PvE arenas.
Edit: To clarify, players who have the mounts already will of course keep them. You just won't be able to get them after 3.2 if you don't have them already.