This probably means you'll be able to submerge with your mount. In one of the older patches in The Burning Crusade, flying mounts were adjusted so that you are no longer dismounted when coming into contact with water. That also made it impossible to dive into the water without manually dismounting from your flying mount. A different behavior will probably happen for ground mounts. Thanks to the change, we can happily cross this off the list of annoying things in the game.
Patch 3.1 brings us Ulduar, dual specs, significant changes to all the classes, and more! We've got you covered from top to bottom with our Guide to Patch 3.1.