They would put people in a comp with Linux and tell them to try and do some simple well defined tasks. While they were doing them, they were beeing recorded by a camera and had they're actions in the desktop recorded as well.
I must say that that is extremely funny. You can see the facial expressions people do, unneded movements of the mouse they do for no aparent reason, et cetera. It is really funny!

The site where you can download the videos of many people is here:
http://www.betterdesktop.org/wiki/index.php?title=Data
You can also check some of the conclusions they reached after the studies on that data.