Wii MotionPlus is an undeniable step-up in terms of the accuracy of the Wii remote, yet it’s a valid point to say that such technology should have already been available in the controller since the Wii launched. So, why has Nintendo taken an initiative to increase the “power” of the controller? The makers of Darwin are making a case that Nintendo is launching MotionPlus as a response to their product. Speaking with GameSpot, Motus CEO Satayan Mahajan explained, “I think it should be a lot closer to the Darwin [Wii MotionPlus]. It’s still missing the magnetometers, so it’s still a system that needs that IR strip to figure out where it is, whereas the Darwin is completely self-contained. If I’m not mistaken, there were a number of postings and blogs, and we’ve been hearing through our friends and family, so to speak, that the Wii MotionPlus was a response to the Darwin.”

I honestly don’t think Nintendo would be threatened by Motus and Darwin. Nintendo has been known for peripherals/add-ons and the big N is in a very comfortable financial situation right now.
Source nintendo everything.