2017-02-22 - There is a permit needed for visiting Arunachal. Got it from Arunachal Bhavan office in New Delhi. The official there asks some questions before issuing one, no charge.
However, the permit checking seems to be very lax. At the sole checkpoint on the overland route to AP, a police officer stuck his head into the bus and called out "permit? permit?" No one got up, he left. No further check points after that for the two weeks I was there.
Arunachali people are of a very different appearance from neighbouring Meghalaya or Assam. The population density is much lower as well. Tribal affiliation is very strong and not easily apparent to outsiders.