Some relevant data:
http://www.flotherm.com/technical_papers/t281.pdf
Skip to page 12 unless the math is of interest. Note where the probes are arranged in space. Then check the baseplate temperatures for those probes with the copper and aluminum heatsinks. The heat just doesn't seem to spread all that much to my eye. Is there another phenomenon here that I am not aware of? The only thing I can think of is the 100lb force, but I don't expect that to significantly improve the lateral heat transfer through the IHS.
Also note that, when lapped, Intel IHS has been shown to be copper.
//edit: My earlier comments were not true. I was looking at their pretty colored heat distribution graphs and must have gotten the aluminum and copper heatsinks confused or something. Sorry for the confusion. The copper heatsink does spread the heat out more laterally if you look at those simulations.
The copper heatsink demonstrates (edit)more (/edit) heat spreading than the aluminum one.
If you can come up with some good evidence that the heatspreader can evenly distribute a CPU's heat to a large region then I would be glad to seriously read through it. It doesn't seem reasonable to me with what I have read and seen though.