I seem to be having the same or simular issue. I've installed ZendOptimizer-2.6.2-linux-glibc21-i386.tar.gz, I've modified the php.ini to remove:
[Zend]
zend_extension_manager.optimizer=/usr/local/Zend/lib/Optimiz er-2_6_2
zend_extension_manager.optimizer_ts=/usr/local/Zend/lib/Opti mizer_TS-2_6_2
zend_optimizer.version=2.6.2
zend_extension=/usr/local/Zend/lib/ZendExtensionManager.so
zend_extension_ts=/usr/local/Zend/lib/ZendExtensionManager_T S.sond/lib/ZendExtensionManager_TS.so
I created a file in php.d called zend.ini and pasted the above into it.
My server and cp run fine however when I go to a site that uses Zend all I see id the page stating that I need to install Zend.
example
http://www.localaffair.com/
My php config can be seen here
http://www.towerhosting.com/php.php
I'm running FC4 with Plesk 8.0.1 everything works great except Zend.
Any help will be much appriciated as I have gone over this several times and can not find the issue.
PS I've also tried ZendOptimizer-3.0.1-linux-glibc21-i386.tar.gz and same issue.
EDIT: I am now on v3.0.1, still same issue.