The Wii Vitality Sensor may be put to good use with a product known as Wii Relax, which Nintendo registered a trademark for in Europe late last year. It has since done the same in these United States.
In fact, it has done so thrice, which is pretty typical for Nintendo. It loves to protect its trademarks over and over and over again, just in case someone else tries to release a "microphone for consumer video game apparatus, adapted for use with TV receivers only" under the name Wii Relax.
Most of what Nintendo has filed the Wii Relax trademark against is the typical stuff, like "video game software" and "computer game programs". Others may offer better insight into what Wii Relax will be, should it ever actually come out.
That includes "parts and accessories of consumer video game apparatus, including those with pulse-measuring functions" which would likely be a good indicator that the Wii Vitality Sensor will be used. It also includes "providing inter-connections between handheld game machines with liquid crystal displays via wireless communication", opening up the door for interaction with the Nintendo DS/i/XL.
Of course, none of that is a guarantee that Wii Relax will actually have any of those capabilities or that Nintendo will even release something named Wii Relax. But they've got the trademark covered.
If you'd like to read the full triple trademark, do so at the US Patent and Trademark Office.