Stranger things have happened in the past. But, I do not think it will "kill the Wii". Both consoles offer different game experiences, covering different markets. As previously stated, one of Nintendo's biggest assets is the Mario brand - a widely universal character and series alike available on a single platform, or at least through a single console manufacturer, Nintendo.
Microsoft and Sony have released very good products for the flag-ship consoles, both improving on some issues with the Wii. For example, the PlayStation Move is much more responsive to actions, at least for me and my family. Although, I can't help but think that it will go the same way as the EyeToy: a superb product, but didn't last all that long (to my memory). It's odd how ideas go 'full circle' and return to us in different formats/mediums - they know they've got an idea, we, the consumers just have to embrace it; just like we have 'traditional gaming' with the PlayStation and the Xbox.