I'm not setup to patch at the moment, but when playing with drupal 5.0 and the cvs version of CCK the nodereference module is not using the correct function name. module_exist() has been changed to module_exists() in 5.0. There are 3 or 4 references to this in nodereference.module.
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Comment #1
yched commentedThanks - I submitted a fix in http://drupal.org/node/78825#comment-152992