I got a Blizzard email this morning from my ticket:

Answer:
There is currently an issue where the Paladin Epic Riding mount cannot be learned without 225 Riding (Expert) if the spell is learned outside of the quest -line. We are working towards correcting this. Please be aware, the Paladin quest is available for completion. For any information or updates concerning this issue, we encourage you to visit our Bug Report Forums, located at www.worldofwarcraft.com/forums.
I'd started part of the Paladin quest before they announced it would be trainable. :S