Sony and Microsoft will release their next consoles very close one from the other as, in this generation, they dispute in the exactly same spot of the market. Clearly, they both want to extend this generation for many years. They did a strategical mistake by spending too much money in development and releasing a very expensive consoles that took years to reach profitability. Even now, it is likely that both companies lost money in this generation so far. The release of Kinect and Move will give, at least, a couple more years to the xbox 360 and ps3. I would say 2013 or 2014 is what they are planing right now, although Nintendo may rush them to another round sooner.
Nintendo, because of the whole different approach, is free to chose when and how they will do it. Probably when the Wii sales stagnate, which may or may not be tied with an imminent Sony's console launch. As they are releasing the 3DS in this fiscal year, I guess they don't plan to introduce a new platform next year, which may cannibalize some of their portable sales. Thus, late 2012 is good guess for their new system.
