This may explains why Barry was able to work around it by setting 'PATH' in the caller's environment however I am not clear why the call didn't work on my system when /usr/X11R6/bin was in the PATH for all users!
Just because your user shells all have a particular PATH set doesn't nescessarily mean daemons will also have the same PATH - it depends on how and where it's set, and how the daemons are being started. Try executing env get PATH in an nscp or a simple debug page, and see if the value returned is what you expect it to be...
