Thanks to a nice article from HowToGeek.com [3], you now have a step by step tutorial of how to hack a Ubuntu install via a password reset ;) [4]
Recap:
Note of course, that there are other ways of doing this, especially when you are using some of the security related distros [5]
Links:
[1] http://pulse.veltsos.com/category/it/software/software/os/linux
[2] http://pulse.veltsos.com/category/it/security
[3] http://www.howtogeek.com/howto/linux/reset-your-ubuntu-password-easily-from-the-live-cd/
[4] http://linux.philosweb.com/drupal/files/image_2_102.png
[5] http://linux.philosweb.com/drupal/node/483