The following changes are live on the Public Beta and Private Beta today (and will be live on the default install when the rest of the changes from this week ship):
Changing keybinds now no longer requires that the new key be unused
If binding a key that is already in use, it will set the other use to 'Empty'
This should make key rebinding much easier as it no longer requires that all new binds are not in use elsewhere
Unbound keys now display as 'Empty' and a notification sound is played when a key is unbound
The 'reset all keybinds' button has been moved to the top of the control mapping UI