Why is this happening? I can think of three reasons (all touched on by the article and discussion):
1) The real collapse of housing has occurred in the suburbs. So people who have lost homes and people who might have considered buying homes in the suburbs are moving back/staying in the city proper.
2) Economists have a term called agglomeration externalities that is a bit of a catch-all term for all of the benefits of a lot of and/or concentrated economic activity. Central cities might be a bit more recession resistant due to the concentration of population and economic activity.
3) Many cities have gone through a central city renaissance making them more attractive at the same time that dense living has become a bit of a virtue (or at least an ideal), and at the same time that economic factors (high gas prices, for example) have increased the cost of living in far out suburbs.