It's too close to next gen for them to bother now, especially now that there's been a healthy buzz about Sony already finding a proper suitor to replace their Cell processor for the PS4 -
http://www.tgdaily.com/games-and-entertainment-features/45212-report-sony-eyes-multi-core-cpu-for-ps4
We're days away from 2010, and Nintendo still has some big titles to put out on the Wii that are crowd favorites (Zelda, Star Fox), plus there is plenty of space for games to make more use of Wii Motion Plus, which has been out for about 6 months now. This should allow them to coast easily into 2011-2012 before releasing a new system, which happens to fall into their traditional 5-6 year plan between home console releases.
The Carnival of Shadows - Folk Punk from Asbury Park, New Jersey
http://www.thecarnivalofshadows.com