I understand that the ground bar may be part of the neutral bar, but when are they required to be seperate?
Thanks
Erol Kartal
They must be separated anywhere down stream of the cabinet with the service disconnect (and connection to the ground electrode).
Thank you Greg and Paul. 
Paul: Here is a question that has come to mind. If the connection to the sub panel is not in conduit, and if the sub panel is itself non-metallic, do the same (or other) hazards exist if the sub panel neutral and grounding terminals are connected?
You are still carrying circuit current over the grounding conductor and voltage will be applied to that conductor at the load end. That can result in a “tingle” voltage if you are sitting on a conductive floor or dirt, using a metal cased tool. That also will not trip the GFCI since the fault path is via the grounding conductor.
Thanks Greg for taking that one…I did not come online yesterday as I am in more pain than I expected at this point in my recovery…thanks again.
Interesting - since I know where such a subpanel exists (100A on a second floor). I think I will go see if I can measure any voltage between the neutral/ground and a nearby copper water pipe.
Second question - which revision of the NEC provided that subpanel neutral and ground busses be separate? In other words, would you expect to see 3-wire feeders in homes older than a certain age?
Frank,
Remember they are connected back at the main panel, the intent of the seperation at the " remote distribution panel " to remove parallel paths and potential objectional currents in the system.
If you remember back in 1996 I believe the 3 wire installation to Ranges and Dryers was nixed.....and so on...the only exception that allows the use of 3 wires for this installation ( ie: sub-panel ) is 250.32(B)(2) which for your FYI is finally removed and not allowed on new installations when the 2008 NEC is adopted in your area....until then if the electrician can meet the requirements of 250.32(B)(2) then to detached buildings the 3 wire allowance will be allowed.....
Now…I dont now any dates because I am not a NEC Historian…I only stay up to date on cycles I teach…1999-2008 NEC…but you will see it in older and newer installs simply because of ignorance…nothing more.
I think the idea of not regrounding the neutral is as old as the NEC but the exception for the ranges came during WWII.