According to Go Nintendo, the rumor is that Nintendo is banning people online who use Action Replay codes. The codes in question are:
National-Dex (After talking with Game Director)
Catching All 493 Pokemon
If it's true, I'm glad. I have Action Replay DS, but I only use it for fun things, like letting me race on all courses in Mario Kart DS online. I wouldn't dream of using it on Pokemon Diamond and Pearl for cheating or for anything on that game. I'm glad Nintendo has decided to ban people online because of it.