An entirely inappropriate bow, and I say that as one who loves Japanese customs. This guy has no clue. Actually, it looks more like a prostration in terms of what would have been appropriate.
1.) Strange angle, many are saying the combination of tall Obama and short Akihito is the cause. 2.) Most see this as a sign of respect, that Obama is a very humble guy. And that he's earned a few brownie points for this.
A bunch of people over there are pointing out how President Bush tore the suit sleeve of the japanese prime minister the last time they met by pulling his sleeve as he walked away. Here's a pic(though it looks photoshopped):