I am going to be contrary.
The NEC does not require 4 conductors to a subpanel in the following circumstance:
A detached structure with no other metallic paths between the structures.
The house is clearly a structure detached from the pole, so the main panel in the house _could_ be fed with a three wire circuit, with ground and neutral bonded in the house.
That said, there is quite likely a metallic path between the structures, specifically the RMC. With the metallic path, you are required to use a standard 4 wire feed to the subpanel. I would not want to trust the RMC as the ground return path, however. We are talking about 100 feet of steel buried in soil essentially as the main ground bond for the entire house. Is there any evidence of corrosion of the RMC?
IMHO there is also a very real chance that the RMC is _not_ continuous, with short lengths simply used to provide protection for direct buried cables.
-Jon
