Receptor Desensitation -- Need explaination at atomic/molecular level
describing mechanism causing a receptor protein to lose activity
(reversibly/temporarily) and sometimes permanently/irreversably with
repeated activation. What causes the receptor to shut down temp and/or
permanently

Several mechanisms.

Most depend on the change in response within the cell to on-going
signaling.  This can result in the build up of a negative feed-back
system.  Possible ways that this might work include modification of
the signal transduction channel response, alteration in transcription
of the receptor, or, increases internalisation and degradation of the
receptor.

Check out this article:http://www.biochemj.org/bj/322/0001/bj3220001.htm
It deals only with G-protein coupled receptors, and others types
(notably RTKs) may be different (or not :-).