Kinect will have a very small impact on Nintendo at best.
Hardware-wise, casual gamers are unlikely to buy several consoles. And they either already have a Wii, or if they don't they are still more likely to buy a Wii because it will stay the king of casual games.
Software-wise, Nintendo doesn't have to worry much either. Most Wii games that sell well don't even work properly with Kinect due to the device's limitations - for example NSMB, Mario Kart, SSBB, Mario Galaxy, Zelda... The only successful Nintendo games I can think of that could be copied for Kinect are Wii Sports (Resort), Wii Fit and Wii/Mario Party. Kinect might have a small impact on sales for those games, but that's about it.
I think it's more likely that Kinect will be losing much of its steam. Even the analysts expected that Kinect will start with massive sales, and that 2011 will show what's really behind the hype, when the initial fascination has worn off and people want to see good games for it.
