In the final episode from last season, the voice of Red John is Eric Stoltz.

I have perfect pitch and easily recognized him.

He hasn't been introduced as a character yet.

Just a thought but when CBI discover the body in the bathroom in S1, maybe the writing says 'He is Mar' not 'Man'.

When Rebecca goes to see Bosco in hospital to shoot him, she say's hello to a man called Marty and smiles.

My theory is that Red John is actually no single person but is an organization with insiders in the government (e.g. FBI agents, and others) in various industries.  There was an RJ industries in one episode.  The organization has a tremendous amount of resources and interests.  The "Red John" murders have been carried out by different people in the organization.  The murders have been committed for fun, revenge, intimidation, or other reasons.  I also suspect that the organization is working on powerful mind control techniques.  In one episode, a woman in black went into a hotel room to kill Hightower.  When she was discovered, she jumped out the window to her death.  She must have under some kind of mind control to do this.  Also, when Christina was discovered in a catatonic state, it is likely that the organization did some kind of mind control experiment on her. 

Arlov. Fits the profile and has been on screen.

Red John is Erica Flynn.. Jane's Irene Adler

Lot of people think Red John is Patrick Jane with double personality. I think is more plausible the idea that he has a twin brother(same smart as Patrick but with an evil side) of course he is not aware of his twin brother, only RJ knows it.
For example if you see when patrick jane make his smiley is quite similar to red john blood smiley, what a coincidence.

Would also explain why RJ save Patrick in 2x23, and he never try to kill him.

At the end of first season, when he kill the cop, before he died, the cop said: red john, jane, shhh (like a secret)

Or when he interview Rebecca, and she said you and red john are so similar.

I am unsure about WHO Red John is but as much as it would be an interesting twist, Patrick Jane as the man is too unlikely. How could he have 'seen' Red John in the episode where he is saved if it is him...

I believe that Red John  will be someone tied to the story in a small way but no major role. This is in conjunction with a pattern that runs throughout every episode where every killer has a small part in the episode, and then is cunningly caught.