There were some additional constraints associated with using the neutral wire as the equipment grounding path from a remote building back to the main panel. One had to do with the absence of any underground metal pipes that ran between the buildings. But I can't find the relevant article in my copy of the 2005. Perhaps someone can chime in with the section.
2008 250.32(B).......
Exception: For existing premises wiring systems only, the
grounded conductor run with the supply to the building or
structure shall be permitted to be connected to the building
or structure disconnecting means and to the grounding
electrode(s) and shall be used for grounding or bonding ofequipment, structures, or frames required to be grounded
or bonded where all the requirements of (1), (2), and (3)
are met:
(1) An equipment grounding conductor is not run with the
supply to the building or structure.
(2) There are no continuous metallic paths bonded to the
grounding system in each building or structure involved.
(3) Ground-fault protection of equipment has not been installed
on the supply side of the feeder(s).
Where the grounded conductor is used for grounding in
accordance with the provision of this exception, the size of
the grounded conductor shall not be smaller than the larger
of either of the following:
(1) That required by 220.61
(2) That required by 250.122
