I expect Wii Sports Resort will feature a lot of pass-the-remote style multiplayer, and very few games with an option requiring 3-4 MotionPluses.
And then hopefully, we just see a few different games which benefit from the controls after that, just like with the Balance Board.
Luckily, third party devs will control whether it becomes standard or not. If almost all games are optimized for NO MotionPlus, it won't become standard.
I don't think the next Mario or Zelda needs 1:1, and I think it would be foolish of Nintendo to try to push this as anything other than another optional peripheral. They've even made an effort to have a no-nunchuk option in multiplayer games.
I believe a MotionPlus costs $30 on it's own.
"[Our former customers] are unable to find software which they WANT to play."
"The way to solve this problem lies in how to communicate what kind of games [they CAN play]."
Satoru Iwata, Nintendo President. Only slightly paraphrased.