They wouldn't have done this for Gore or Kerry. I don't think anyone did it when Bill Clinton was elected, and I'm not sure it would have happened for Hillary had she been the winner. It's not just because of the color of his skin - well, that's a big part of it, I'm sure, and it explains the celebration in Kenya, but I think a lot of it also has to do with his personality, his intellect and his story.
It also has to do with his popular support. Yes, that's sort of circular reasoning (he's popular because he's popular), but stay with me.
In the 2008 election, unlike several previous cycles, the two main candidates appear to have really been selected by the people, not the parties. It seems to me that if the parties had been in charge, we would have seen Hillary Clinton against Mit Romney. But the people, the voters, didn't let that happen. They picked the maverick and the new kid, not the faithful party stalwarts who were next in line.
I think that's a good thing. Political parties aren't bad, but government of the people, by the people, is even better.