We learned from BlizzCon that heirlooms will become truly Bind on Account. What we didn't learn was how that would be accomplished. Bashiok took to the forums to explain.
It's a new UI of (essentially) buttons that will "light up" when you obtain one (or have obtained one) of the heirlooms. Click the button and an item of that type will be generated and put into your bags.
Questions from the forum that he didn't answer were
- What will happen with heirlooms you can wear two of, such as weapons?
- Will the heirlooms keep their enchants?
- Will we get a refund for having more than one of each heirloom?
My speculation is that it will let us generate more than one of the item so that we can dual wield heirloom weapons if we like. I also expect that the heirlooms will not be generated with enchants but that the gear that is already on characters or in bags will remain enchanted. And I expect that just like mounts and pets, we will not get refunded for having more than one of any heirloom. But all of this is just what I infer from the information given, and we'll need to wait and see what Blizzard's exact system is.