OK, so I've been looking for a name for this thing where people graduate towards 2 parties under FPTP and I've discovered there's a law called Duverger's Law (Duverger was a French sociologist). I feel better now as I have a name for it.
If you want to know more, read: http://en.wikipedia.org/wiki/Duverger's_Law. It explains why I have such a passionate hatred of FPTP (because it means that politics remains the reserve of existing players rather than working more like a market).
It's also why nearly all the anti-AV writing out there is by people connected with the 2 main parties.