Ancilorn over at the European forums has confirmed that the armored Frost Wrym mounts mined from early builds of the Patch 3.3 PTR will be rewards for achievements in the Icecrown Citadel raid. Much like past raid dungeons have awarded mounts for completing meta-achievements, such as the Ironbound Proto-Drake for Ulduar, the Frost Wyrms will be "up for grabs," for dedicated raiders.
MMO-Champion previously unearthed this armored Frost Wyrm model along with spells named Frostbrood Vanquisher and Frostbrood Vanquisher Flight, which could give some insight into some Icecrown Citadel encounters. Despite the confirmation of new reward mounts (expected to grant 310% flying speed, as well), there is currently no word on the removal of the Ulduar reward mounts, which Blizzard has said to stay for "the foreseeable future".
Patch 3.3 is the last major patch of Wrath of the Lich King. With the new Icecrown Citadel 5-man dungeons and 10/25-man raid arriving soon, patch 3.3 will deal the final blow to Arthas. WoW.com's Guide to Patch 3.3 will keep you updated with all the latest patch news.