pretty sure i read on the latest update of the app that it had been opened up to everyone.
it's pretty slick, my only complaint is that it seems you cant set yourself as 'offline' ... i dont really want to be messaged on my computer and my phone at the same time ... and i dont want to have to 'log out' on the app whenever i'm at my computer. if i want to chat on my phone, i should be able to mark myself as 'logged in' without having to re-enter my user/pwd