I met a guy about 8 years ago who built and installed PT wood foundations. Since I had never heard of this technique, I naturally asked a few questions.
This foundation type, according to him, was primarily used in Cold Climates because of frost heave and the key to the system was insuring that water stayed away from the PT foundation.
He talked how they made sure that the subgrade/subfoundation had a substantial gravel backfill(12" to 18") and a filter fabric surrounded the gravel to insure the fine particles did not infiltrate over time.
The key to PT foundations is to make sure that the PT lumber stays dry and water is allowed to drain.