afaik its simply the amount of calculation that have to be done for WPA encoding. the DS would a dedicated chip for wpa encrypting.
I'd love such a feature but it propably wont be possible in software.
Ninteno will tell you if you call them that you can use WPA security IF you are using the WIFI USB access point. This is because the USB point connectes to the PC that is inturn connected to the router. So your DS is actually connecting to the Internet via the PC not the wireless network on the router. This will allow you to do whatever you wish on the router and still be able to play online.
That is your only option.
Perhaps you should just enable MAC address security and tell your neighbor's 12yr old kid if he hacks your WEP key you will show him what Hell is like.