In terms of the Wii the impact will likely be so small as to be negligible.
Sure, price matters, but only when comparing like for like. Between now and December 2008 I doubt MS can change public perception of the 360 enough that someone in the market for a Wii would buy a 360 because its cheaper or has a copied Wii Mote.
The 360, for better for worse, is right now locked in a battle with PS3. I'm very confident the majority of Wii purchasers don't even consider a PS3 / 360 when they head into the store. Their choice was made beforehand and is based on market perception and 'zeitgeist' for want of a better term.
I suppose if Wii's were out of stock some 360's might shift as a replacement - but most kids of friends I know would not be happy to find out their desired Wii has morphed into a 360 without the Mario games they want to play.
Or to use a more direct analogy, if I went out to buy an Ipod I wouldn't buy a Zune just because it was in stock and cheaper.
Try to be reasonable... its easier than you think...
