I know this discussion has occured before, but now there is a bit more proof and info to back this theory up. Before I begin, let me first say that I did not believe this until today. I believe that Pein is really Minato, the 4th hokage. The manga thus far has provided more details and facts to support this claim.
1. It was stated that the 4th only sealed half the 9 tails, the light half, in his son, Naruto. Where did the other half get sealed? I believe that due to the fact that the 4th was aware of his power and ability, he felt that he would be able to control the dark half of the 9 tails, and sealed the other half in himself. This could be why the 4th no longer has love for his sensei. The dark power of the kyuubi has destroyed all traces of his emotion.
2. If you look at Pein's new body, it does in fact resemble Deidra. With the piercings, removed, use your imagination, it clearly is a likeness of Deidra. Now, whether or not this is Deidra's actual body, or some clone is up for grabs. But, the point I am getting to is that Pein's initial appearence can also be said to resemble Minato, the 4th.
3. The 4th hokage was known as konoha's yellow flash due to the fact that he created a jutsu that enabled him to travel great distance in a small matter of time, and also because he is a straight up badass. This is not opinion since it was stated that a "flee on site" order was given during the third ninja war when in the presence of the 'Yellow Flash'. Pein has shown the ability to project his image, as well as the other akatsuki members images across great distance, as shown when akatsuki meets. Now, if some one can create a jutsu that teleports themselves great distance, why would they not be able to create a jutsu the projects their image, as well as others, a great distance also? This can be debated, but, I feel that it would be possible.
4. Now, one question you may ask is " Why would the 4th hokage take orders from Madara uchiha, or anyone else for that matter?" Well, first of all, shinobi respect power and ability. If Tobi's ability does in fact prove to be great, then it is easy to understand why a 'dark kyuubi 4th hokage' would follow someone. Uchiha Madara would be the only type of badass that someone like Pein could respect, and follow.
Please feel free to discuss this issue with me. I will think about this matter more, and post my theories here, as well as read others.