As far as I understand, this is because
mod_passenger requires Apache version >=2.2.9-9, but the
apache2-mpm-itk of Lenny is only version 2.2.6-02. This is at least a reported bug, and even claims to be fixed, but apparently has been reported again. Is there any trick to get
mod_passenger installed while keeping the
mpm-itk flavor of Apache? Apparently Passenger would work with Apache 2.2.6-02, the problem is just that the package dependency is set to wrong version.