Even though aluminium is not ferromagnetic, you will still see magnetic flux in the space between the conductors, and thus current flow induced in the enclosing conduit. The flux is stronger when the conduit is ferromagnetic; think 'iron core transformer versus air core transformer'. Clearly the separate conduit approach works; I'm curious as to the details of the engineering to prevent inductive heating.
I encountered this sort of conduit on a tour of the 'Glen Canyon Dam'. This was a standard open tour group, not some sort of special event. At one point the walkway has a very good view of the distribution transformers. Google images found this photo:
You can't see it in the above image, but at the end of the row of transformers (the side nearest to the viewing area), the ductwork is vertical; three _large_ pipes side by side. There was a horizontal rail near the ductwork.
Someone had arranged what appeared to be a coffee can, inverted and free to spin on a spindle, just sitting on the horizontal rail near the ductwork. The can was spinning merrily away. I believe that the can was essentially the rotor of an air core induction motor with two half turn stator windings. I suppose that it could have been wind, but I don't recall much wind at the time, and the can was smooth, so the wind should not have caused any torque.
-Jon
(edited to add image to ECN server)
[This message has been edited by electure (edited 10-22-2005).]