I think I posted this somewhere else in the forums too. But anyway, I had my SSID broadcast off, and the wifi wouldn't connect but the store etc would. I set my access point to broadcast my SSID, and it works perfectly now?!
I had the same issues: would work with DSi apps, but not with Acekard wifi plugin. I had to set my SSID to "broadcast", then it worked great. (I currently have broadcast off and was using mac filtering)