Cable such as this is common in South Africa as it offeres a higher safety factor - a nail can enter from either side and the first conductor it reaches will be 0v (well, close to Earth anyway!).
In fact, if my memory serves me correctly, the twin had to (at one time at least) be wrapped in a conductive foil so that penetration from any angle would contact Earth first. Having the earth wire on the one side ensured contact with the conductive foil (if it were in the centre, there may be a possibility the wire would stay free of the foil).
Marc, I think the South African wire you are referring to is this...
It's called surfex and is designed for surface mounting. Very nice to work with BTW. I hadn't thought about it before but with all power circuits there being on earth leakage units even nicking the neutral thought the aluminium sheath would throw the breaker.