Mounts are truly cross-faction now!
Today I logged into my alliance druid to go do her farming, and my random mount mod pulled up the Spectral Steed. It took me a minute, and then I realized this was a mount I didn't HAVE on any alliance toons. I hit 85 way too late to bother with TBad. I had the Spectral Wolf on the Horde side, courtesy of my druid main.
I mentioned this in guild chat, and was asked if the Chopper/Hog was cross-faction. And sure enough, though I've only ever made the Mechano-Hog, my Night Elf Druid had the Mekgineer's Chopper in her mount list!
I realized I couldn't check the final one, as less than a month ago, I had traded gold with a friend to buy the Traveler's Tundra Mammoth for my Alliance characters (I gave her money horde-side, she gave me money ally-side, and we both bought them), but a RealID friend confirmed that her Alliance characters now had access to the Tundra Mammoth she'd only bought for Horde characters before.
My ally toons also have the lion mount, when no one is exalted with any ally guild, so the Scorpion/Lion also works.