Nintendo announced the Wii MotionPlus accessory is only for new games, there are no plans to retrofit old games:
"Wii MotionPlus accessory is only for games that are designed to make use of its abilities. Wii MotionPlus is an optional accessory for developers and players who want to further refine their game experience," a spokesperson for Nintendo told us. "There are currently no plans to make it compatible with existing non-Wii Motion Plus enabled titles."
The Nintendo spokesperson added that Wii will continue to support games which work with the original Wiimote powers. These will behave no differently when played using the clip-on MotionPlus accessory.