As far as I can tell, it should be the same as for nds and gba development: you can create and sell your homebrew software, made with unofficial tools, putting somewhere a disclaimer stating that you and your software are not sponsored or endorsed by Nintendo. Of course your buyers need a backup enabled console, because you can't write a "genuine" cd.
About becoming an official Nintendo developer, you can check this page:
http://www.warioworld.com/apply/wii.html

The interesting part is that Nintendo sells devkits only for DS and Wii.