Text for the link-phobic:
It looks like those waiting for news of account-wide mounts don't have to wait any longer. The latest iteration of the Mists of Pandaria beta has introduced account-wide mounts on all characters. This means that any mount you currently have on your main will be granted to your alts as soon as they reach the appropriate level to obtain them, as well as any achievements associated with the mounts.
I tested this on a level 85 premade monk as well as a freshly copied-over level 81 druid just to see what would and would not copy, and I was pleased to see that the Amani War Bear from the original version of Zul'Aman made the transfer. I was even more delighted to see that the Violet Proto-Drake, granted from the achievement What A Long, Strange Trip It's Been, was also included in the list. Master Riding, required for 310% speed flying, is not granted along with it. Characters who do not have Master Riding will still have to purchase it from a trainer, regardless of the mount being available to ride.
However, as far as I can see, there are no exceptions to the list of mounts as of yet. Obviously this may change as the beta progresses, but for now, players can look forward to enjoying all of those sweet mounts on all of their characters in the beta.
Edit: It appears that mounting the Violet Proto-Drake on my level 81 without the Master Riding ability gave the character Master Riding. Whether this is something that will make it to live or not, we don't know -- but it's certainly something to enjoy on the beta!
Keep in mind that as with all things Beta, there will certainly be restrictions, like for class and faction specific mounts and things - anything might change, really. I, for one, am completely and absolutely excited for this. Now if I can only get Anzu to *@&%#*@) drop... >.>