Hard to tell at this point in the generation. If Move and Kinect were released in 2008, this question would be easy to answer.
There are many theories for the Wii's year after year losses in hardware sales since 2008, but I have since rested with the theory that the Nintendo Wii, if you look back at the average length of Nintendo console life cycles, coming into it's 5th year is at the end of its' cycle, thus its' hardware sales are showing year over year losses since it's prime time in 2008.
Even during the December of its' 4th year, the Nintendo Wii sold more than any other console. If the Kinect was "eating" into Wii hardware sales, it should have dominated the 1st holidays it was released. After all a platform that sells 8 million in 4 months should push its' console to the top month after month right?
Not according to the US consumer in December 2010. The Nintendo Wii was king that month.
So, no the Kinect is not eating into the Wii's hardware sales. The Wii is on it's last year or two. This is what is eating into the Wii's sales.