I bought a Nintendo DS Lite recently and wanted to try the WiFi funtionality for playing over the Internet.
I have a Linksys WRT54G with DD-WRT as firmware. After some messing around I found that I need to set it to B mode only (so not mixed), with 64 bits WEP. I also entered my SSID statically so that I can keep the broadcast disabled.
After this I was able to connect to my router. I then was able to playing Koi-koi (Hanafuda) over the Internet against some other human player. Quite nice and fast too!