Why those nm numbers? Hmmm... surface density (nm squared) roughly doubles every two years. Slower than Moore's Law (double every 18 months).
Also it's a bit more complicated that just a doubling; again from Wikipedia, the original formulation was "about the density of transistors at which the cost per transistor is the lowest." (http://en.wikipedia.org/wiki/Moore%27s_law#Other_formulation...)
Intel is of course not quite playing that game, with speed and power being somewhat more important than cost.