"The Terrain Camera (TC), a 10-m resolution stereo camera onboard SELENE (KAGUYA), succeeded in imaging the inside of the crater, which was faintly lit by sunlight scattered from the upper inner wall near the rim. The estimated temperature of the crater floor based on the crater shape model derived from the TC data is < ~ 90K, cold enough to hold water-ice."
http://www.sciencemag.org/cgi/content/abstract/1164020

Linked from:
NYT

Which said:
"But images taken by a Japanese spacecraft throw hot water on the idea that Shackleton could have surface ice. Since parts of the crater are in permanent shadow, the images relied for illumination on sunlight glancing off the walls on the opposite side."

Which was linked from:
http://pajamasmedia.com/instapundit/

So, somewhere around 90 K or higher in Shackleton crater. Or around -183 C.
Compared to:
http://www.solarviews.com/eng/moon.htm
"Minimum surface temperature -233°C"

So that's 50 C warmer.
And reminder the sublimation temperature for H2O in a vacuum is -150 C.
I would guess the closer the temperature is to -150 the more likely there is less
ice on the surface [and in general] and would tend to cause ice formations below the regolith.