A recent study in florida showed evidence that urban heat island affect is not only real, but potentially the main contribution of mankind to "global warming".

http://wattsupwiththat.com/2009/02/10/floridatrend-its-hot-but-dont-blame-global-warming/

Notice that where there was no substantial development the temperatures actually DROPPED.  I realize Florida is only a small part of the planet but if this were the case worldwide, the reality of "global warming" would be radically different and totally incompatible with many of the extreme environmentalists views.  If land use were our only significant contribution then most alternative energy projects would actually make matters WORSE! 

Switching to alternative energy sources means we would stop burning coal, oil or using nuclear power in "small", conveniently located plants.  Instead, power plants would be moved far outside the cities and would consist of expansive facilities of metal and concrete...greatly increasing the urgan heat island problems.