Wierd. I checkd the Nintendo DS wireless page a few weeks ago. It listed my D-Link 514 router as incompatible. Now that I have Mario Cart - I'm testing it and sure enough it is incompatible. It's a standard 802.11b router - and I did try it with all of the security settings off, broadcast SSID on etc.
Does anyone know what non standard/strange feature the DS uses that makes it incompatible with some 802.11b routers?