Here is Ed Glaeser describing the recent economic successes in Manhattan and in Santa Clara. He describes both as successful "cities". Glaeser has published many fine studies that call attention to the benefits of density and citihood.
I had already blogged here about research with Sandy Ikeda which pointed to success in the same two places as indicating that the simple consideration of population density tells us very little.
Clearly information networks matter a great deal. Such networks grow in a variety of urban settings. It's pretty clear that the future will bring many more Silicon Valleys than Manhattans -- and Manhattan is very likely to continue to adapt and hang on.