Nintendo today announced the Wii MotionPlus add-on for the Wii Remote. I'll let the Nintendo press release explain it:
The Wii MotionPlus accessory attaches to the end of the Wii Remote and, combined with the accelerometer and the sensor bar, allows for more comprehensive tracking of a player’s arm position and orientation, providing players with an unmatched level of precision and immersion. Every slight movement players make with their wrist or arm is rendered identically in real time on the screen, providing a true 1:1 response in their game play.
In other words, you move your arm forward six inches and your character on the screen will do the same. You can see this level of precision being used for games like Splinter Cell, where you'd have to pick a lock ever so carefully, or, perhaps, any number of object manipulation puzzle-esque games.
This upgrade to the motion sensor will help to keep Nintendo in front of any motion device that Microsoft or Sony may announce.