The Wii U console will ship with a sensor bar identical to the one for the original Wii, but it won’t be required to play Wii U games. The Wii U sensor bar is only required to play old Wii games, which only rely on the Wiimote controllers.
The GamePad controller doesn’t need the sensor bar to function. However, for multiplayer games that require Wiimotes, such as Nintendo Land, New Super Mario Bros U, and others, the sensor bar is needed in order for the Wiimotes to function properly.
But if you’re only into singleplayer games, you can essentially leave the sensor bar in the box for now. Furthermore, Nintendo revealed in a patent a few months ago that the GamePad itself can be used as a sensor bar. Nevertheless, Nintendo is including a sensor bar with every Wii U bundle.