stewey, one other reason is that Manhattan is a lump of rock, so it's easier to build skyscrapers. There's a fair amount of subsoil on much of it but the rock is close to the surface at two places, Midtown (where the Empire State building is and right Downtown (where the World Trade Center was). (For the same reason, I don't think they have any deep-level underground trains, too hard to tunnel.) Skyscrapers on London clay need a bit more support. But Ken Livingstone loved them and I think Boris does too, so they will continue to be built - when the money is available. A lot of them have been put on hold because of the recession. A good thing too as they are actually planned to look more like Shanghai than Manhattan, all wacky shapes - mobile phones, shards of glass, cheese graters and so on.