My cutsom error page only displays when you add .asp to the end of the URL string. You can test this at the site below:
http://ww1.sph.uth.tmc.edu/dhgfdjfgdj.asp <--- This will work (custom error msg)
http://ww1.sph.uth.tmc.edu/dhgfdjfgdj.htm <--- This will not (generic error msg)
http://ww1.sph.uth.tmc.edu/dhgfdjfgdj <--- This will not work either.
Windows 200O Adv Server, SP4, IIS 5. Would really be interest in your thoughts