Obama presents brilliantly - young, articulate, great personal story - of which his color is a part. He is following a President who, however unfairly, is less popular than Nixon. People often get very caught up in 'now'. Don't forget that a recent poll very nearly voted the pretty but largely worthless Diana as the Greatest Briton of all time (a last minute campaign by engineering students got the great Isambard Kingdom Brunel up ahead of her). If Diana can come close to this, then all bets are off.
Right now people are making him into what they want him to be. Having done very little actually works for him at this point - people can still make him into whatever they want. It won't last. Youth & charisma are attractive - thus figures like Kennedy, Che, Jim Morrison & Jose Antonio have been idolized as ideal types for different groups. Obama plays that role now, but unless he dies quickly he will get tarnished. Politics has a way of giving peope very sticky clay feet.
Popularity & achievement don't always go together. Churchill may have been a great (if flawed) wartime leader, but he didn't win his first election for PM until 1951. He got the job in 1940 without ever going to the people, and in 1945 & 1950 the people he had led through war voted against him. They saw him as a man for a particular time & place.
History has judged Churchill well. Obama will have his chance. I doubt he will end up anywhere close.