Nintendo say that the Wii MotionPlus (pictured below) 'redefines game control, by more quickly and accurately reflecting motions in a 3-D space. This means that movement of player's arm should be captured more precisely during games.
The blurb on the official site reads:
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. The Wii MotionPlus accessory reconfirms Nintendo’s commitment to making games intuitive and accessible for everyone.
Be sure to check out our full E3 2008 coverage