Evidentally there is the potential for a faulty a/c adapter. However it could be something wrong with the Wii itself. Also make sure its got room to ventilate and that the fan runs occasionally. One change that may be worth a try is to put the wii on its side if it isn't already like that. If the Wii is on its side, it receives somewhat better airflow for the internal cooling fan.
This is what Nintendo recommends trying before calling them, but if it does not work you'll notice theres a customer service line at the bottom. Hope this helps
Originally Posted by Nintendo.com