Call-time pass-by-reference

I won't pretend to know exactly what this means (my guess is that it is a PHP5 related issue), but I get this warning:

Warning: Call-time pass-by-reference has been deprecated - argument passed by value; If you would like to pass it by reference, modify the declaration of taxonomy_save_vocabulary(). If you would like to enable call-time pass-by-reference, you can set allow_call_time_pass_reference to true in your INI file. However, future versions may not support this any longer. in /var/www/vhosts/baade.org/httpdocs/sites/www.baade.org/modules/gradebook/gradebookapi.module on line 63

While this is not in itself a problem (I modified my php.ini) many users on virtual hosts may not be able to make this change.