Via Gamasutra, an interview in USA Today with Nintendo's Reggie Fils-Aime confirms that the Wii's online component will be free to play. The outspoken Nintendo advocate says
"We will offer online-enabled games that the consumers will not have to pay a subscription fee for. They'll be able to enjoy that right out of the box. The Wii console is going to be Wi-Fi enabled, so essentially, you'll be able to plug it in and go. It won't have hidden fees or costs."