Not really sure, but all DS/D lite model worked with WEP only. That was a feature I would have liked the DSi to solved, but obviously it did not. I never found any information related to any different encryption for the DSi, so that may be still the same Chip/firmware doing the encryption. (Hey, Nintendo, wake up, WEP have been broken almost 10 years ago!)
I would suggest you give a call directly to Nintendo support, they sure know.