i wish to discuss about the use of Aluminium foils under the heatsink

i've came across some manufacturers like Dell, and XBox which uses aluminium foil in between the heatsink and CPU

http://www.joystiq.com/2006/03/12/mi...eatsink-video/
http://www.youtube.com/watch?v=B-wyBafJpSI
http://www.hardwarezone.com/articles...91&cid=14&pg=2

also .. some Intel CPU stock heatsinks are equiped with it as well

any ideas why they're placed that way? could it improve the temps?
i could not find any relevant articles regarding to this alu foils under the heatsink