It's just a matter of the Zend extensions not porting correctly after the upgrade of Plesk.  When you install Zend, it ends up creating a symbolic link from php.ini to its own located in /usr/local/Zend/etc, thus the real file is backed up and no longer exists.  The problem is, when Plesk upgrades, it overwrites that symbolic link with an actual file, thus all of the Zend configuration options are effectively gone, this causes Apache to crash on startup when the modules can't find their specific configuration lines.

What you need to do is, uninstall any of the Zend products, whether it be Zend Optimizer, PerformanceSuite, ServerCenter or whatever, upgrade Plesk to 7.1, and then reinstall those applications...OR...backup the configuration, note the changes that Plesk 7.1 makes, and then port those changes over to the new configuration file, making sure the symbolic link is recreated, and the changes are made to php.ini within /usr/local/Zend/etc

I will be making a post to this effect on your forums, so that other users will be able to see exactly what to do to fix it.  Please keep this in mind, as 7.1 is an awesome upgrade, and it really took me by surprise that there were issues, because I know Plesk quite well.  I am sure I am not the only one that uses Zend...so you will probably see this arise as more and more people try upgrading.

I hope this helps you guys out.  If you have any questions, don't hesitate to ask.