I wish people would stop comparing the Kinect to the Wii. They are really not that similar, besides having a similar lineup of 3rd party shovelware. Wii genearlly represents unique ideas in games, as well as old school gaming values and characteristics. Kinect mainly has a bunch of "birdmen" games with limited gameplay, which is also why only a few of the games are selling significant numbers.
And to answer the question, no, they are not eating into Wii sales. Nintendo has hurt its own sales by focusing too much on niche hardcore games for most of last year, as well as third parties cutting the life of Wii short from very little support.