Freezing headset cups to ease installation - does this actually work?
According to this link the thermal expansion of aluminum 6061 is 13 micro inches/(inch x °F).  A headset has a diameter of 1.125 inches, and circumference of 3.5325 inches.  Assuming you change the temperature of the headset from 70°F (room temperature) to 0°F (typical household freezer), you'll change the circumference by 13 x 70 x 3.5325 = 3215.5 micro inches.  Which is .0032 inches. The diameter will now be smaller by .001 inches.  I really don't think that will be enough to make a difference when installing a headset.  Hopefully somebody can check my math/science to ensure I'm applying the information correctly. 

Here's a nice chart illustrating the thermal expansion of aluminum from 0 K to 293 K (20°C).  This seems to fit with my numbers from above that shows a decrease of 40 degrees (+20°C to -20°c) results in a change in length somewhere in the vicinity of 1x10-3 length units.