Rumours from GDC this week suggest that Nintendo are considering shipping the MotionPlus device inside a Wii Remote jacket.
According to IGN, the idea came up several times whereby the add-on motion sensor would be stuck inside a jacket, essentially forcing players to use protection, funnily enough for that extra level of sensitivity. It's likely that it's a case of miscommunication, where what's really meant is that Nintendo would ship the device with a spare jacket (as it would need to be somewhat bigger than the standard ones).