OS: Ubuntu 6.06
Cacti 0.8.7e
Plesk 9

Setting up cacti went fine until trying to run the poller:

# sudo -u cacti /opt/psa/bin/php-cli /var/www/vhosts/hostname/httpdocs/poller.php
sh: /php-cli: No such file or directory
sh: /php-cli: No such file or directory
sh: /rrdtool: No such file or directory

Had to modify the php.ini (/etc/php5/cli/php.ini) for the php-cli binary:

;safe_mode = On
safe_mode = Off

The cacti poller did not like safe mode being enabled on CLI PHP binary.  After this was changed, cacti started polling properly.