That percentage was grabbed out of the ether. I've no idea about his chances.
What do I mean by "Great President"? Franklin Delano Roosevelt is considered a "great president," but he was obviously still an capitalist-imperialist president. But so far as US presidents and capitalist politicians go, he's considered great for what he accomplished, and what he did was rise to the occasion. He responded to two gigantic crises with what turned out to be great competence.
Obama isn't on his way to being a great president. He's a bunch of empty words, stirringly delivered, who has surrounded himself with the same old batch of failed imperialists and con-men and con-women. He's been a disappointment precisely because he promised so much change, but has offered mostly betrayal. I was going to check out a recording of his inaugural address on Crooks and Liars because I was so impressed with his delivery at his victory speech, but he thanked bush II for "his eight years of service" and I couldn't stomach anymore.
But Obama has the same chances that FDR had. Gigantic world crises, including an even greater bankruptcy of the capitalist system than the Great Depression. Unlike bush II, Obama is an intelligent man who can respond to reality. He'll be a "great president" if he's able to abandon the failed wisdom of the Washington system and respond with sensible, pro-people policies, because that's what's necessary, not increasingly desperate and obvious servitude to oligarchy.
On the other hand, maybe it's like Alexander Cockbourn put it in his own impish way. He loved bush II because nobody did a better job of destroying the American Empire. Maybe a complete failure would be the best thing for the world.