If a helmet cracks, it's absorbed energy, cracking is simply how it's designed to dissipate that energy. Whether it cracks or not, it should be replaced when it absorbs energy from a fall.
Cracking, in this case, is the expected failure model, since the material has to be hard to provide a sliding surface.
Helmets should not be re-used after a crash.
http://www.bhsi.org/ideal.htm point 5 - outer shell