What's the real cause-effect?
Correlating liberal voting with urban population density is one factor, but look a little closer. In Arizona, for example, I'd say the two blue areas are indian reservations (entitlement lifestyle, IMO, although not urban). Yet, Phoenix (Maricopa county) went for Bush. Other metro centers in the solid-red heartland did not go blue.
So its possible to have major dense urban populations who don't vote liberal. Can anyone put their finger on a characteristic that would separate, say Phoenix from Los Angeles? Can we cultivate these characteristics (in terms of public policy) to promote less-liberal (dependency) behavior in big cities?