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.