After I posted the comment for the post below I went on to search again and lo and behold..
"A tower of unprecedented scale conceived not as a building so much as a vertical extrusion of the city - a new town in the sky complete with parks, public squares, schools and hospitals. At 1500 metres high (the average level of cloud cover), the tower would create a new and completely different scale to the existing city forming a separate layer superimposed above London's ancient and idiosyncratic street plan."
For more info : http://popularchitecture.com/supertower/
While we're at this, check out Taisei Corporation's X-Seed 4000!
Designed to be 4000m tall, with 800 floors that could accommodate up to 1 million inhabitants!