I doubt it will be that significant. Japan has been at AA- from all rating agencies forever (and has vastly higher debt) and pays lower rates than the US. Besides, as you pointed out there are so few large countries with AAA from all agencies.
Besides, there's no new information released today, other than what some analysts at S&P think.
Japan is a nation of savers and has a massive domestic pool of savings. Also, Japan will drop below investment grade within the next ten years as those savers retire.
America, on the other hand, needs to borrow internationally.
Japan's famous savings rate, just like its famous work ethic, is nothing but a myth anymore. It stands now at 2% in 2009. (and its famous work ethic...no more. 30% of its working population is part-time. look up 'freeter')
Beyond that, I'm not sure why any of these agencies has any real credibility anymore, since they were giving AAA ratings to the "toxic" CDOs up until they collapsed.
Besides, there's no new information released today, other than what some analysts at S&P think.