I'm not sure if this forum topic is still open, but I do know that a LOT of people have been having problems with this. I appear to have found a solution.
Go into your device settings and delete ALL network connections, including the ones related to your DS connection (Its a separate setting from regular internet, and can be found in the network settings)
Once that is accomplished, re-enter the network information for your wireless internet into both the aforementioned areas, ensuring that all the information is 100% correct. Then, just to be certain, power-cycle your DS.
To test that everything is working, try using the DS Internet browser, Nintendo eShop, and the friends' list. If all of these function properly, then you're done! If one or more of these aren't working, double-check that your network connection settings AND your DS connection settings are the same. Fair warning: DS connection is only compatible with WEP security, so if your router uses any other kind of security you'll have to remove the password and try again.