[Soul Surfer]

The average temperature of all the material in the earth is not very interesting but the temeperature at the surface due to internal heating in the absence of the sun is quite interesting. 

This could be estimated by lmesiring the heat flow out of a square metere of soil. you can get this from the average temeperature rise as you go deeper into the earth and assuming that this has to be radiated into space.

It will be pretty cold but definitely well above absolute zero or CMB temeperatures my guess is probably in the range 100 to 150 Kelvin.