Wow. I have missed a lot over the last few days. Sorry to pick up on topics that have sort of been and gone, but I'm not convinced that Kate has extensions. I have extraordinarily thick hair, which can be hard to manage, especially when I was younger and probably not getting the best cuts, tools etc. So my hair was heavily thinned, at my request, and I used straighteners to make it very smooth, and fairly flat (even if I left some of the natural curl).
Then DS came along. head of hair that has to be seen to be believed. Anyone who has known my family, knows it comes from my side - thick, slightly coarse, and a LOT of it. But everyone assumed it was DH. So I got very petty annoyed and told my hairdresser to stop thinning my hair!! 
It took about a year, but my hair is now a LOT more like Kate's hair is now. If I let it get even longer and absolutely stopped all thinning, I think it would become even more similar. And if I get it blow dried, it can absolutely do the big swishy thing (I just don't like the big swishy thing. On me, or Kate!).