JBD and ptonsparky, I think you are agree without knowing it. You will need three conductors, but only two phase conductors. One can be the neutral or a grounded phase. Either way, you save some money on the insulators.
In theory, you could use the earth for the neutral, but I don't see why you would. Where there is only a single phase, it makes more sense to use the earth because you won't have to worry about conductors that clash => longer spans => cheaper.
[This message has been edited by C-H (edited 11-17-2005).]