Vne is a structural limit, Mne is a limit to stop the airflow over the top surface of the wing becoming supersonic. As you climb the local speed of sound decreases (speed of sound in air being solely(well almost) dependant on temp) at some point the Mme limit will become more limiting than the Vne limit which remains the same throughout.
HTH,
TFS
just re-read johns post, i think ive probably repeated exactly what he's said, oh well never mind................