There's a pretty interesting blog post at http://contentconsumer.wordpress.com/2008/04/27/is-ubuntu-useable-enough-for-my-girlfriend/. I have not read through all of the comments yet (510 and counting). After a brief scan of the main article, it occurs to me that the test would be interesting to run on a machine with Microsoft Windows Vista and on Mac with Apple OS X. Of course each time you would have to find someone that isn't particularly educated on the operating system they are being tested on.
Might be fun. What do you think? Maybe I should test my wife on my MacBook Pro to see what happens. Maybe not.