I would play DS online much more often than I do, but my router is set for WPA security and really don't want to lower it to the inferior WEP everytime I play a DS game. Thankfully, the Wii is WPA compatible so that will not be an issue. With any luck, the Wii can act as a special DS-only access point, though I doubt that.
Also, the range for online access is really terrible for my DS. I have a nice Linksys router and everything, but I can't get access outside of my room. Heck, I think certain areas of my room are off-limits too. Granted, my router is pretty tucked away.