Hmm. My mistake. Here it is. Read for yourself: http://forum.outerra.com/index.php?topic=2022.msg21732#msg21732
Ah yes thanks for the link, now I remember having seen this, but the model is of a very low & "bad" quality...
One can surely manage good LOD properties on very large models, using tools like simplygon.com
but for a complete city, I wonder, because if we want to have a convincing representation of such a city, close to the real place, at low altitudes or at ground level, then it means a lot of facets and pixels.
Today it's easy to find this kind of accurate representations of large urban areas, the trick is to extract vector datas from these huge models for procedural usage (generation of roofs, facades, roads, poles, etc).
By the way there is a lot of research on this : of course it is feasible by hand (but so time consuming) so if it would be automatic, with smart recognition + extraction, it would solve the issue.