a router has to be public and open and be a WEP otherwise you need to enter the security key into the DS.
The first part is true. But for those with a WPA Key on their Wi-Fi cannot access the Wi-Fi on their DS system. I found that out the hard way. In short, I had to secure my wireless connection and I heard that WEP isn't as good as WPA because it is easier to crack. So I decided to use WPA and I then could not access the WiFi on my DS. After looking it up, Nintendo Wi-Fi does not support WPA.