From the 'Pedia:
According to the Nintendo Style Guide: A Guide to the Proper Usage of Some of Nintendo's Products:
|It is simply Wii, not Nintendo Wii. It is pronounced "we", indicating its all-inclusive nature. The name works best at the beginning of declarative statements. For clarity, it is best to avoid passive verbs and prepositions.|
So, it's not the "Nintendo Wii". South Park's usage hasn't improved matters.