I had occasion to install Drupal 7 on my own Ubuntu server recently. Many of the guides I found went into some quite unecessary steps, involved liberal use of tasksel, or made a variety of assumptions about your environment. In the end, though, I found this guide, which is short, simple, and just worked.
The only addition I required was to run a2enmod rewrite and to edit /etc/apache2/sites-available/drupal to have AllowOverride All in the entry for the /var/www/drupal/ directory.
Job done in almost no time.