Seeing as the Sony Sixaxis has motion sensitivity as well (though not the the extent of the Wii), it only make sense that Microsoft would work on this as well.
Being the only one without it is yet another thing they missed by trying to get the drop on everyone.
It wont encourage anyone to buy it.
People who want a 360 will buy a 360, people who want a Wii will buy that.
I doubt it will sway anyone away from Nintendo.
