[jefftk]

The subfloors I've seen for old houses in Boston are also laid perpendicular to the joists, though diagonal does seem like it would make more sense.

[hinkley]

Laying a board over a seam in the subfloor sounds like a titanic pain in the ass. Diagonal means you might have to move the nail a little. Probably also limits the amount of dust and water that passes through from floor to ceiling.