"We’re pleased to announce a convenient upgrade to the way that mounts and vanity pets are handled in Wrath of the Lich King.
Players will be able to “learn” a mount or non-combat pet much like learning a spell, recipe, or new ability, and these creatures will then show up on a new Pet tab within the Character Info section of the interface. Players will be able to access and preview their learned mounts and vanity pets through this tab.
Once learned, the pet icon or mount icon will no longer appear in bag spaces or bank spaces. This inventory space will be made available once again for other adventuring needs. Pets can still be set to hotkeys by dragging them to the hotkey bar, much like any other spell or ability."
Some new information coming out of the WotLK beta, and a blue post. I think this is a great change, as it will open up at least 3 slots in my bag (vanity pet, ground mount, flying mount), and alot more in others.