I went through three months of discussion with Apple top level technicians.
The Final result was admittance that this is an SMC problem.
(I was given an Apple Extreme Base Station free for my trouble)
The end result was an alteration to SMC but only for the newest Mac Pro and not previous.
(see Apple updates for the 2013 Mac Prohttp://support.apple.com/kb/DL1724 )
In few easy to grasp words....
The internal bus speed of the newer Mac Pros occurs so fast that low speed USB connections (your keyboard) are skipped. The System Machine Code (SMC) does not receive a connection handshake within a certain time period and determines that no connection exists. (stops trying to connect)
Oddly enough this does not occur in all models due to slight grounding differences present in cabling and or computer or monitor. (those well grounded work just fine)
ALL do not have this problem and it is somewhat rare but certainly very real.
For those of us who have this problem and do not own the newest Mac Pro with SMC update, there is currently no solution except to disconnect / reconnect keyboard upon startup and restart.
(the Mac Pro will maintain USB connection if put to sleep and woke from sleep)
Somewhat humorously.... if you connect a wired mouse to your keyboard, it will make USB connection with the mouse but not the keyboard it is attached to.
SIDENOTE:
If your monitor / display System Preferences do not show a Brightness Control Slider, you have a secondary problem that is fixed with the EDID_Reset_Tool.
This problem is typically associated with a repair of the monitor. (it is part of the Apple Hardware Test AHT that is now obsolete)
An internet search for your monitor model will give you this tool. (Must be for the correct Monitor type)
For the 27" Display Port Apple monitor.... http://download.info.apple.com/Apple_Hardware_Test/EDID_Reset_Tool.dmg
The only other option is to run an online hardware diagnostic. http://support.apple.com/kb/ht1509
PS
My monitors have always been too bright, not too dark. (creates false photo print adjustments)
My settings are one notch above zero brightness.
If you monitor is too dark, odds are.... your room is too bright. (in energy physics law and engineering, this condition is called a Bias created by the photoelectric effect)