Despite Wikileaks saying the Japan-US cables will "rock" Japanese politics, there doesn't seem to be much new in the leaked US cables about Japan.
Revelations so far:
1) Hatoyama and the DPJ knew they were never going to relocate Futenma, they just stalled on saying so to keep the coalition with the Social Democratic Party together.
DPJ government never committed to Futenma alternatives
http://www.asahi.com/english/TKY201105040063.html
2) the US used China's military build up to argue against Futenma relocation. The only thing shocking about this is that the US government usually doesn't openly mention China by name in these kind of discussions. Kind of a moot point now after the Senkaku incident.
U.S. used 'China card' to thwart Futenma alternatives
http://www.asahi.com/english/TKY201105040065.html
3) The numbers of Marines that need to be relocated and the cost of that relocation were manipulated for political reasons on both the US and Japanese side. The Okinawan governor called the figures B.S. at the time and turns out he was right.
Numbers inflated in Marine relocation plan to increase political impact
http://www.asahi.com/english/TKY201105040066.html