mod_rewrite was not enabled, by default, on my Apache installation. I am on Ubuntu 7.10 (which is in development, at present). To enable it, I simply issued the following command:
sudo a2enmod rewrite
and then restart apache with:
sudo /etc/init.d/apache2 force-reload
To disable it, simply
sudo a2dismod rewrite
and then restart apache with the above command.