An Asian site reports the CPU automatically shuts down at -88°C and that it can't even boot at -10°C:
Apparently the CPU won't boot if it's colder than -10 degree C and it will shut down if it hits -88 degrees C. If you're playing with liquid nitrogen, this is definitely going to be a major issue and even people with cascade coolers will have problems.
Saying that, even someone that's got one of Asetek's Vapochill cases will run into problems here, as they might not be able to power their system on after a reboot. The question which remains is to see if this will affect other 45nm parts from Intel or not and it's something that extreme overclockers won't like.