Apparently Nintendo did not anticipate how hard you guys play Wii Tennis. After several accidents Nintendo takes action and recalls/replaces the 0.6mm strap and replaces it with the apparently much better 1mm strap (see photo).

Nintendo says if you use the strap appropriately the 0.6mm model is also save. If you feel you are too strong you should call Nintendo to get a replacement strap.

This is possibly the cheapest recall in history for such a mass market product. It is just a strap. AP reports that the Wii strap recall will cost Nintendo only $1 million.
The details on how to retrieve a new strap for the US have not been released yet. More details in this Nintendo announcement (Japanese).
More Nintendo Wii News.