||01-08-2011 10:28 AM
I'll add a little more to what JP said.
The neutral, which is correctly called the grounded conductor, is a current carrying conductor and has current flowing in it any time you have a 120 volt circuit active.
The ground, correctly called the grounding conductor, does not normally carry current. The only time this sees current is in a fault condition which is why it is allowed to be bare within the NM cable.
If the neutral and ground were tied together in the sub panel, which wire would carry the 120v neutral current?
Your main panel is different because it is connected directly to the POCO transformer with three large wires. Your house is really a MWBC so the only current on the neutral, it is a true neutral at this point, is the imbalance between the two bus bars.
The neutral and ground bus are tied together and connected to the GEC. One thing this does is reference your system to earth potential. The POCO transformer probably has a ground wire from the secondary center tap to earth ground too so it is reference to earth potential at that point also.