From a brief Nintendo statement:

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.

The company promised more details at its own press conference Tuesday morning. But its pre-announcement this morning suggests Nintendo thinks Microsoft is going to introduce some kind of motion-sensing controller. Nintendo likely wanted to get out ahead, and show the hundreds of game industry reporters here that it's now on its second iteration motion controller.