Prediction: there will be no clear ending to the generation. The next microsoft system could come 2010 or 2011, and the next sony system might not come until 2013 or 2014. In essance this will be the first time we have overlapped generations: Nintendo and Microsoft on one scale, and Sony on another. Sony could release a PS3 slim with a fast loading blu-ray drive to counter Microsoft's next system. Over the long haul there's really no way the PS3 doesn't sell more than the 360, but it will be on a longer timescale.
Sales prediction with the last year they are still relevent in the market
Wii: 130 million through 2012
PS3: 90 million through 2016
360: 60 million through 2012