Hmm I think first Sony is working on the PS4 right now. So they will be prepared to release their console as soon as MS did it. They made even a survey what the developers want in the PS4 and we had a lot of rumors yet.
Second MS has lost 5 Billion on the gaming devision and they stat to gain back the money the shareholders would like to see keeping the xbox360 as long as possible so will Sony.
MS said weeks ago that they dont even now if the 360 reached half of its lifecircle.
Second they release natal in the end of 2010. They probably want to market this thing for more then just one year. SO 2012 is the earliest moment.
But I think they will try to keep the gen going as long as possible so they can make the money back they lost.
I think this gen will be the longest of all.