That's the whole point of it, you can lock or keep it registered with as many devices as you desire. But as most only stick with one device it can be either way.I would say no to this because I like to log my accounts on different devices sometimes. I do like Vold's idea even though it would be a pain after getting a new iDevice and already having to wait a long time for download and patching.There should be an option where you can lock your account to one device (Preferably your main device). So that when someone else knows the password, user and try an log in, it will fail.
This is the best way to prevent so-called 'password guessing' and 'hacks'.
I like the facebook idea too, but then again it has a lot of twists to it like having the facebook account guessed! Where as the simple device account suggestion has no twists other than just keep your device in your hands. You can give your password to 10000 of ppl but they wont be able to log in because their devices are not registered with the account.
Again, the point is for the person who has access to it can register with any of their devices. (More than one i guess)