According to the Sandia's Cooler whiter-paper pdf: http://prod.sandia.gov/techlib/access-control.cgi/2010/10025... the limit is due to a "Thermal Brick Wall". Basically due to a lack of progress in heat exchanger technology.

I have visualized this "Brick-Wall" as a graph with data from Wikipedia pdf: http://dl.dropbox.com/u/3215373/Thermal-Brick-Wall.pdf