I used the previous installation instruction on Ubuntu 12.4, and the php-curl module is successfully installed, (php-curl used in installing WHMCS billing System):
sudo apt-get install php5-curl
sudo /etc/init.d/apache2 restart
By the way the below line is not added to /etc/php5/apache2/php.ini config file as it's already mentioned:
extension=curl.so
In addition the CURL module figures in http://localhost/phpinfo.php
Best,