I can't see the Wii under 80 million under any scenario. They have the momentum, they have the wand, and there's not much software yet (relatively speaking). As said previously, Nintendo would actually have to blow up for a sales collapse!
PS3/360? I am not so certain of a confident minimum. That darned Wii wand makes a big difference! Even with price drops, the PS3/360 still don't have Wii appeal.
My question is "where will the "traditional, mainstream purchasers go?".
Fall 2007 will go a very long way is describing that.