Yeah, it's true. I had forgotten that mythology.
For those who doesn't know, here's a recap :
http://en.wikipedia.org/wiki/Ten_spiritual_realms
Six realms of desire
All of Pain's realms were taken from the 6 realms of Desire of Buddhism.
The six lower realms are Hell, Hunger (Ghost), Animal, Anger (Asuras), Humanity and Heaven (Devas).
Hell (Hell Pain)
Hunger (Ghost Pain)
Animality (Animal Pain)
Anger (Demon Pain)
Humanity (Human Pain)
Heaven (God Pain)
There's also another theory regarding the 4 Nobles realms.
Some might see some correlation between Naruto's life and those realms:
Four higher (noble) realms
The four higher worlds are characterized by the belief that humans need to make an effort to reveal themselves from within their lives.
Learning
Learning is a condition in which one seeks some skill, lasting truth or self-improvement through the teachings of others. To access this realm, the experiencer must first develop the desire to gain wisdom and insight into the true nature of all things, free from delusion. This realm is characterized by the seeking of truth and wisdom through external sources, e.g. other people and pre-recorded information (usually texts).
Realization (or absorption)
Realization is a state in which one discovers a partial truth through one's own observations, efforts and concentration. Usually to access this realm the experiencer must first have decided external sources are inferior to internal sources, eg his/her own mind. This realm is characterized by the seeking of truth and wisdom through direct internal perception.
Bodhisattvahood
Bodhisattvahood is a condition in which one not only aspires for personal enlightenment but also devotes oneself to relieving the sufferings of others through compassionate and truly altruistic actions, e.g. helping others. This realm is characterized by the feeling that happiness achieved through the benefit of others is superior to happiness achieved through the benefit of only the self.
Buddhahood
Buddhahood is the highest of the Ten Worlds, a condition of pure, indestructible happiness which is not dependent on one's circumstances. The experiencer is totally free from all delusion, suffering and anger. It is a condition of perfect and absolute freedom, characterized by boundless wisdom, courage, compassion and life force. This realm is difficult to describe and is generally only obtained through the direct internal perception of the realm of realization. This realm is characterized by not being shifted into lower realms due to external sources, and the non-reliance on external sources for happiness. This realm is manifested outwardly through the actions of the realm of bodhisattvahood.
So, seeing how Naruto is now trying to save the world by himself, I think he's already at the Bodhisattvahood realm.
I like this theory because it works well, but I wonder if Naruto will actually get to the Buddhahood...