No, it is not standard unless you have FC5 installed. CentOS 4.x comes with PHP 4.3.9 and Plesk is designed to work with that PHP version on CentOS 4.x by default.
It is quite easy to install PHP5 on CentOS 4.x - just change the "enabled=0" line to "enabled=1" in the "centosplus" section of /etc/yum.repos.d/CentOS-Base.repo yum repository definition file and then run "yum install php" to upgrade php from 4.3.9 to 5.0.x
One thing I must mention about Plesk 8.0 and PHP 5 - if you have Plesk PowerPack and use or plan to use Application Vault feature, then you should be prepared that you won't be able to even install half of the applications from Plesk app. vault.