Plesk is installing apache, php, bind, mysql, etc from the vendor's repositories. Those packages are maintained by the vendor in the normal way (yum, up2date, etc).
Upgrading PHP is not an issue with PSA, I have 5.2.6 rpms available for CentOS 4/5, RHEL 4/5, and Fedora 4-9 now. Apache can be a bit more complicated, provided you are sticking with the major release for your distribution (IE, Apache 2.0 for CentOS 4, Apache 2.2 for CentOS 5, etc) you won't have any issues.