We've created some installation instructions for Ubuntu on the Gumstix Overo.  It's a mashup of some other tutorials that results in a working installation of Ubuntu 9.04.

http://johnwoconnor.blogspot.com/2009/04/installing-ubuntu-on-gumstix-overo.html

Also, the previous post has some images.  Video to come.

http://johnwoconnor.blogspot.com/2009/04/ubuntu-904-on-gumstick.html