i'm a RAW shooter. I have lot of experience with the 5D, 7D and A700.
Long exposure noise reduction on those DSLR's (so i guess on all Sony DSLR's) just shoots a black frame for the same exposure time as the RAW exposure. Then the camera substracts any noise found in the black frame from the RAW.
This feature helps eliminating noise generated by temperature changes of the sensor due to long exposure.
E.g. taking long exposures in a cold winter night of the sky. This feature is typical used by astrophotographers.
If you are shooting long exposures under conditions with minimal thermal differences between the ambient temperature and temperature within the camera, you can turn off the long exposure noise reduction. E.g. Indoor shooting.
Wish you all good light
Carlos Di Vega