One thing that a colleague here raised was the prospect of one device
with several network interfaces on it. In this case, the device is going
to have to keep track of which interfaces are active, and return EBUSY
as and when needed. This doesn't strike me as a Bad Thing.

One thing that I don't handle yet is the way an interface is implicitly
brought up when you set an address on it. At the moment it appears that
device drivers are responsible for knowing that an interface is
supposed to be brought up when an address is set up it. This doesn't
strike me as being logical, but fixing it the way I think it ought to
work would affect userland stuff, and then we would get on a makework
slippery slope.

-- 
Lloyd Parkes
"For my confession they burned me with fire
 And found that I was for endurance made." - The Arabian Nights, trans. Haddawy