One reason was because of this comment by the Japanese Prime Minister
Standing beside Mr. Obama at the Japanese equivalent of the White House, the Kantei, Mr. Hatoyama said, “We’ve come to call each other Barack and Yukio, and gotten quite accustomed to calling each other by our names.”
To westerners this may not seem very significant. So they're on a first name basis, so what? But in Japanese culture, referring to someone by their first name is extremely rude unless you are close friends and of similar social status. For example, a Japanese colleague of mine felt extremely uncomfortable when his English supervisor referred to him by his first name. Even though he was on good terms with his supervisor he felt that it was socially unacceptable to have his professor (someone of higher social status) refer to him by his first name under any circumstances.
Its similar in French where, as I understand it, 'tu' is used as a friendly term while 'vous' is more formal situations. You would not refer to the Premier as 'tu', but as 'vous'. I remember reading a biography of Bourrassa where it was pointed out by the author as being a significant when journalists stopped using 'vous' in addressing their questions to Bourrassa as it demonstrated a loss of respect for him.
In any case, the fact that the Prime Minister of Japan refers to Obama by his first name is significant for perhaps two reasons. Without trying to go too deeply into it, the Japanese Prime Minister did study at Stanford, so it could just be that he recognizes Western culture and understands that westerners see being on a 'first name basis' less seriously. But it could also be taken as an astute political move to demonstrate to the Japanese people that Japan is not a subservient nation to the USA. That the Prime Minister of Japan and the President of the USA are not of unequal social status, which would line up very well with the positions that Hatoyama has taken recently, asserting independence from US policy to an extent not before seen. I could be wrong, but I don't recall reading anywhere that Bush and Koizumi, despite being called 'BFF' by some in the media ever publicly stated that they called each other by their first names.
So while this might be an exercise in putting on a good show for the media, I'm tempted to believe that it reflects Hatoyama's fundamental desire to re-work the USA-Japan alliance to one where Japan is considered more equal.