It's still a bit too early. Nintendo should ride out the poor sales of Wii for awhlie longer, at least until 3DS is out and the hype has settled down. It has built up enough of a lead that Nintendo doesn't need to rush the next console, even with poor sales.
The reason being, I still don't feel the full potential of the Wii has been realized yet. Wait for Zelda, bring out some more motion plus social games and Vitality Sensor. Also, Nintendo still has room for a $50, or even $100 price drop, as they are making a nice profit on the Wii.
Now, If those fail to reignite the console, THEN it is time for the next one. But they should ride this out until at least early 2012, as the holiday season will erase much of the defecit Wii gains during the slower months.
2012 should STILL be more than enough time to race ahead of the competitors, as it looks like Sony is content on riding out the PS3 for as long as possible, and I don't think they're in any financial state to throw out a new console right now anyway. And MS has reignited 360 sales at least in the US with Kinect, so I don't see a new console coming from them anytime soon.
2012 would be a good strategy for Nintendo, because by that point they will have built up a strong enough lead on the Wii so they could not be even close to being passed, AND Nintendo would almost ensure themselves 1st place for the NEXT generation as they would have a huge headstart.