Most of the questions regarding wifi (from various threads, hence the new one) are covered by this short paragraph from nintendo's contact us site:
Sorry, no link as it was part of an automattically generated help page.The system boasts 512 megabytes of internal flash memory, two USB 2.0 ports and built-in Wi-Fi access. There will be a number of Wi-Fi-enabled launch titles, currently in development, that will employ Nintendo's wireless gaming service, Nintendo Wi-Fi Connection. In addition, there will be no charge for Wi-Fi connections, or first-party Nintendo wireless Internet content.
You will still need the nintendo wifi connector for your pc to actually connect to the service (or a standard wireless router presumably).