Php 5

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.

Getting PHP5 working w/PLESK 8.

You might actually need to do a little more.

I wasn't able to get PHP5 running using just the above instructions due to a php-xml dependency. So in addition to the above, I also upgraded php-domxml:
yum upgrade php-domxml

Then ran the php upgrade:
yum update php

Then rebooted the server. Whatever you do DON'T remove php-domxml. If you do, you'll kill your PLESK installation...

I haven't thoroughly checked everything out yet, but all the services still seem to be running fine, and I have PHP 5 on the system now.