From the Asus OC Guide:
(Please take this with a grain of salt)
Q: How does it do Under Liquid Nitrogen?
A: As the Core frequency scaling capability is inversely proportional with respect to the change of
temperature when the CPU temperature goes down too low, the overclock capability of the CPU
actually reduces dramatically when it reaches below zero degrees. For example, a CPU may do
5.0GHz @ 0C, but only 4.9GHz @ -40C.
The ideal temperature for overclocking the Sandybridge
processor under the P67 platform is around 15~20 degrees.
OCM