I bought my son a NintendoDS this weekend along with Mario Kart. I searched Google for a while, trying to figure out why I couldn't see my AirPort with the DS and Nintendo's wifi support site doesn't work from a Mac-based browser.
Anyway, all I had to do was set the Multicast rate to 2 instead of 11. Just figured I would pass this on in case anyone else runs into this.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060403071934631