Enable mod_rewrite on the fly o Ubuntu

Enable it:
sudo a2enmod rewrite


Reload apache2 to enable it:
/etc/init.d/apache2 force-reload

Disable it:
sudo a2dismod rewrite

source:
Configure mod_rewrite in Apache2 on Ubuntu 8.04

Dont forget to change AllowOverride None => AllowOverride All /etc/apache2/sites-available/default file.