Hopefully Legendaries will be account-wide if they're no longer tradeable. I think this is a horrible idea though as the player market is already so tightly bound by high static prices and insane market tax. I love player driven economies, so what they're saying about it all really depresses me.
But if I can't mail my Legendaries between my characters, I hope they'll be account-bound then and I hope they'll give notice on what they plan to do here so, if I have to, I can move them where I want them before they're stuck there forever.
I miss open markets in games, even if it invites real-money trading from third parties, I think it hurts more to not have the system than to have it.