A workmate has drawn my attention to a new tool for constructing 3d city models: CityScape 1.6 by PixelActive. Although I had no time to try the demo yet, it seems to me that the included models fit best for cities in Norther America. The supported file formats include elevation data and Collada, which makes it interesting for city planners, too. Another interesting feature is the real-time animation. However, my guess is that it is most useful for game designers.