A musical instrument is basically an object that produces sound.
From this view a controller can never become an instrument, but may well be part of it.
For example, a traditional piano has a controller part (keys) and a sound generator (strings).
The main difference of electronic instruments is that both parts are not necessarily in the same box.
@Tim, aha - semantics! Join us, over on the 'Misc' forum.
@broc, yeah, the piano is a tricky one due to that mechanical disconnect (I'm not a pianist), but I have no doubt that concert pianists feel an intimate connection with the sound-engine part of the system, thereby transcending the controller 'interface'. Also consider, for example, the violin. There is a clear yet intimately cross-coupled connection/relationship between the controller (bow, fingertips) and the sound engine (strings, body). I'm really just shooting the breeze with this one; pop over to the 'Misc' forum to join the party.